As the title says, I'm not the most skilled League player, but I've spent about 8 hours watching videos on rune optimizations after realizing that some runes I was using were just straight bad. I wanted to summarize and share that knowledge with the community because I haven't seen anyone do this yet. Also because after learning the new system the game stopped feeling so unfair (the stuff you see in rant posts lately) and I started winning more.
There are some runes as I found out myself, that just aren't worth it in almost any case. I will try not to tell you to straight up never take them though. The season is still early, and there may be use cases yet discovered. I will also try to point out the use cases even if they are barely used or have no optimal uses/users. I will also try to make this a short read, but I've learned I'm pretty bad at that.
* Press the Attack - Good for auto attackers and people with auto-resets. General case best in slot. Requires that you can get 3 autos off in ~90% of fights. Sustained damage option.
* Lethal Tempo - Good if you have an ability that needs a lot of autos to get high damage (think Kalista's Rend). Worst in slot, no optimal users. Requires that you are an auto-heavy reliant champion. Suggested champions where it comes close to other choices in certain cases: Kalista if you want to focus on Rend, Sivir if you want to focus on Ricochet spam, _maybe_ Jinx in certain matchups/playstyles only(might just be fun instead of close to optimal), _maybe_ Aatrox with certain playstyles(get W off quicker after engaging), _probably_ Xayah. Typically for non-single target damage, usually runaan abusers, or for secondary affects like Rend, feathers, or Aatrox healing (because if you're doing it for damage on W, Press the Attack is better).
* Fleet Footwork - Good for short trades in lane, hard matchups in lane; essentially for sustainability in lane. Good choice if your main focus isn't damage. Best on poke lane comps.
* Overheal - Good for shield/heal stacking comps. If you plan to get a Bloodthirster and have a shield/heal support (preferably shield, or Locket) on the team (and are an ADC) then you should consider this rune. Again, requires that you buy into Bloodthirster and have a shield on your team that you can be given to be optimal.
* Triumph - Good if you want sustainability from this slot. Also helps with the gold. General case best in slot.
* Presence of Mind - Good if you are taking Precision primary and are heavily mana-gated. Also good as a secondary rune if Precision is your secondary tree and you need mana. Not many users for marksmen where it's optimal, because most marksmen aren't that heavily mana-gated, would recommend considering more as a secondary. Good choice on Sivir for Primary, good choice on mages for secondary along with Coup de Grace in the bottom row.
* Alacrity - Good if you need attack speed or are taking Precision Primary. Best in slot, straight up higher gold value than the others. Wouldn't recommend taking the other two unless your champion values those stats much higher than most.
* Tenacity - Worth taking sometimes, generally best used as a secondary on a melee champion (personally I like stacking tenacity on Dr. Mundo, although that doesn't necessarily make it optimal). Take on champions you value tenacity on who can't use attack speed well.
* Bloodline - Almost no optimal cases. Has by far lowest gold value in slot. Would recommend taking only for niche/fun builds where you want to stack a lot of life steal.
* Coup de Grace - Good for killing low health targets. General case best in slot. Would recommend sticking to this rune in this slot unless you want to change according to the enemy team comp.
* Cut Down - Good for killing high max health targets like tanks. Would recommend taking if the enemy has a lot of tanky champions (2 tanks at least, probably near optimal if they have 2 tanks and a high-ish healththird champion). Personal suggestion: take as secondary on non-marksmen(mages, bruisers, tanks) if tanks are a concern, ignore on marksmen and build Lord Dominik's Regards instead (seems likely optimal usage, although I have no higher opinion to back this up, purely personal speculation).
* Last Stand - Good for champions that can maintain a low health level in fights and enjoy extended fights. A bit of a risky rune, as most champions that fit that description can be rather feast or famine. Would recommend considering them on Nasus, Warwick, Vladimir, (new Swain?), Darius?, current Irelia, Kled, Maokai, and Mordekaiser?.
* Electrocute - Good if you are looking for the most amount of burst damage you can get in the game. In lane it is worse off than Thunderlord's Decree because of higher cooldown if you fail to proc it quickly once it comes off cooldown. In the jungle, you are proccing it at about the same rate due to clearing jungle in between ganks, so it is a straight buff to burst as a jungler. Optimal on burst junglers and good on roaming burst champions.
* Predator - Good if you value the movement speed and are confident in your ability to properly use the rune. Fair amount of burst. Only rune that requires large amounts of gameplay and strategy on your part to use (high skill ceiling). Can't really say any optimal cases as I just don't know, but conceivably good on roaming champions, movement speed reliant gankers, and hook supports (suggested strategy of take it on hook supports, buy boots first back, roam mid at mach 5 grab the champ and lock them down till they're dead). Again, high skill ceiling, wouldn't recommend taking if you aren't confident you can use that.
* Dark Harvest - Good mid game and late game power, but gives up a lot of power early. Generally only good on junglers, although if you roam and farm a lot, conceivably good for that champion as well. Requires that you farm jungle/gank optimally. If you consistently get less farm or kills than the enemy jungler, I would not suggest taking this rune. Need to reach 150 soul essence as fast as possible, if you can't get it by around the start of mid-game consistently, I would not recommend.
* Cheap Shot - Good if you can consistently slow or hard-CC an enemy. Need to be able to proc the rune multiple times per trade for it to be worth choosing. Not garbage if you can only get it once per engage though. Generally for champions/builds that lean towards poke.
* Taste of Blood - Good if you want some sustainability. Most champions can proc the rune when it gets off cooldown on a consistent basis. Highest gold value (by only a bit), and general case best in slot.
* Sudden Impact - Good if you have at least two of the following: stealth, dash, blink, leap, teleport. Not too bad if you only have one though. Should be taken on a champion that usually buys/considers penetration in their build. Should be able to do full combo/bulk of damage _after_ proccing, and within 5 seconds it is active. Generally for builds/champions that lean towards burst.
* Zombie Ward - Good if you destroy a lot of wards (buy a sweeper), or require a little more vision during laning phase. Optimal uses are on supports, green smite junglers, and mid laners. Also good on top laners (although generally they don't want this tree). Would not recommend going domination for this rune, but instead to go domination for a different rune and get this one too, if you fit the optimal cases.
* Ghost Poro - Good if you want vision and roam/invade a lot. Not much to say beyond that, offers less vision radius than others, but will always be available. Please note though that if you plan to hide in bushes in wait a lot as part of your strategy, that it might not be what you want as that will move the poro.
* Eyeball Collector - Default if the other two aren't appealing enough. Also good if you want to do as much damage as possible, although, vision is usually straight up better.
* Ravenous - Good if you value spell vamp. Choose on champions that like Hextech Gunblade or Death's Dance. Also good on high sustain champions. Not recommended to choose on champions with multiple AOE abilities, or where their AOE is their highest damage ability (1/3 effect on AOE).
* Ingenious - Good on supports and champions with either multiple active items or a single active item that their build/playstyle is built around. Works on trinket cooldowns and vision is very valuable.
* Relentless - Good on roaming champions or junglers. Would recommend on champions that don't value the other two and want Domination primary. Would also recommend on champions who buy Dead Man's or Righteous Glory (although you should also consider Ingenious for Righteous Glory).
* Summon Aery - Good for poke champions and shield/heal support champions. For poke champions/builds you have three choices of keystones: Fleet Footwork, Summon Aery, and Arcane Comet. Choose Fleet Footwork if you want to play safer, Aery if you have no slow/hard cc naturally in your kit or if you also heal/shield, and Arcane Comet if you do single target damage mostly and have a hard cc/slow. Aery is optimal on damaging/poke/mage supports that also heal/shield like Soraka, Janna, and Sona (although Guardian is a strong contender). Aery is also optimal on mid-laners like Malzahar who have no single-target or non-DoT abilities. If you take Aery it is suggested that you have at least average movement speed and have the capacity to get a bit closer at times so that you can reduce it's cooldown (meeting back up with Aery).
* Arcane Comet - Good for poke champions. Specifically poke champions with single target abilities. You should try to look at the champion you are playing and min-max whether you can reduce Arcane Comet's cooldown significantly. If you have mostly AOE or DoT abilities, probably choose Aery. If you don't have a slow or hard CC, probably Aery. Simplified general cases: If you have a hard CC/slow, choose Comet. If not, Aery.
* Phase Rush - Good for champions that have reliable damage as long as they can stay close, but may have trouble staying close. You will need to be able to hit three separate abilities, attacks or a combination of the two (all separate) to be able to proc. Notable users: Singed, Darius, Cassiopia.
* Nullifying Orb - Good if you want a defensive option. Especially good if the enemy laner primarily deals magic damage or if the enemy team deals mostly magic damage. Remember it only procs if you are low health like Maw does, and like Maw only works against magic damage.
* Manaflow Band - Restores mana, makes a spell free currently. Choose this rune if you need mana sustain on a regular rotation throughout the entire game, or if you can abuse it to bully lane early on. Will be changed soon [TM] to be less abusive, so keep in mind it should ideally only be taken to bully early or in lieu of one of the mana items in your build (so that you can build more damage).
* Ultimate Hat - Choose if you value ult cooldown over mana use if it's your primary. Other reasons to choose is because you have a high utility ult (think Ornn ult, Zilean ult, Malzahar ult), or if it has a really high impact in the playstyle of that champion, without making it available more than it can be used (for example Nunu has a high power ult, but getting ultimate hat makes it up more often than you can use it without it being interrupted immediately).
* Transcendence - Good if you want to do a build that has more CDR in it than you can use. This rune in the average game comes on around 15 mintues. The average game length is 30 minutes. Please note that for the average game you will be playing without a rune for half the game. For supports, the average game will have it come on around 18 minutes (60% into the game). This is very much a late game rune. On top of that it has the lowest gold value in slot unless you reach 10% CDR over the cap in your build, at which point it has the highest gold value. You should really only choose this rune if you are in the Mid, Top, or Jungle roles; planning on building for late game; and want to make an item build that gives too much CDR and would otherwise be sub-optimal because of it.
* Celerity - Generally best in slot. Choose if you value the movement speed. The AP/AD it gives is not a lot, and only matters a bit for last hitting, and for champions that get very fast (Hecarim, and Rammus' Powerball). Most marksmen want this to some degree for kiting, and it is optimal in slot for them.
* Absolute Focus - Good rune for tanks and champions that can stay near full health for a full rotation. I say it's good on tanks because the amount of AP/AD they get from it is kept the longest (as they are tanks) and is a highly significant damage increase for them as it may be the only bonus AP/AD they have (besides style bonus). May also be good on skilled assassins who can get full rotation off before taking a lot of damage, and may be good on some skirmishers or fighters.
* Scorch - Good for poke champions. Generally good on most champions, although optimal on champions that can routinely proc it as it comes off cooldown.
* Waterwalking - Good for junglers. While it is always worth considering on a jungler, I would suggest it in particular for fighters and slayers. I would also suggest considering it on slayers you take in lane, if you plan to roam with them. There also may be cheese implications for coordinated teams if you can force fights in river often.
* Gathering Storm - Not a bad choice on any champion. Please note however that it gives nothing for ten minutes (1/3 of average game length) and that for most champions Gathering Storm is worse than Scorch if you can get Scorch off relatively close to when it comes off cooldown, at least until 30 minutes (when the average game ends). Gathering storm is, for the same reasons as Absolute Focus, most significant on tanks. Choose on champions that can't get scorch off consistently and that you don't want Waterwalking on, on tanks, or if you feel especially confident you can stall out a game.
* Grasp of the Undying - Good if you can stay in combat for a long duration on a consistent basis without dying (even without grasp). Best on people looking for durability who want to be in the center of a fight, but have no reliable hard cc (think Dr. Mundo or other Juggernauts in particular as well as some tanks). Generally speaking, if you find Aftershock's effects appealing, but can't use it, you want Grasp. If you want to play really safe/defensive in lane so you don't lose, it isn't a terrible choice on an ADC either (although damage-wise not optimal).
* Aftershock - Good if you are a champion who wants to survive for as long as possible in the center of the enemy team, and has a hard CC. Not recommended on any ADC, mage, or squishy.
* Guardian - Good if you are a shield/heal support who wants to stick to their ADC, are a warden-tank, or a champion that buffs other champions (Nunu, Galio, Braum). Would not recommend on any ADC or traditional mid-laner (although it might be fun to try on Kindred with Ultimate Hat).
* Unflinching - Good on melee champions who are susceptible to CC. Most notably the fighter class are good users of this rune, and I imagine some slayers might like this rune (although not necessarily on this tree). Should consider if you are going Resolve primary and the enemy team has a lot of hard CC or if you plan on getting Mercury Treads.
* Demolish - Good for split pushing. Would recommend taking on melee champions who are good at split pushing thanks to their kit (auto reset, Rammus R, Tryndamere R...). Would not recommend if you don't plan to split push, and would inform your team early on that you will be doing that at least a bit because of Demolish. Scales with health, so best to buy at least one health item, and not to take on squishies.
* Font of Life - Good if you are a tank that wants to stick with the team for fights and consistent slow or hard CC. Should be stacking health if you get this. Best case scenario: High health tank with AOE hard CC/slow that teamfights well.
* Iron Skin - Good if you are playing bot lane or against a strong early game AD heavy opponent. Also good if you scale off of armor and are an early to mid game champion.
* Mirror Shell - Good if you scale off of MR and are an early to mid game champion. Also good if you are fighting a heavy magic damage enemy in lane who is strong early on.
* Conditioning - Gives the best resistances of the three, but doesn't come online until ten minutes (1/3 of average game). Take if lane is easy for you, or if you are a jungler (minimized risk).
* Overgrowth - Good if you build several health items and aren't the jungler. A jungler should not take this rune because you get significantly less health from it by being in the jungle or river. Likewise, the more you roam (or are out of lane), the less you will get out of this rune. It is actually a really low gold value rune, giving only a little more than a ruby crystal worth of health to a health tank by 30 minutes and having less before that (so it's worth less than a ruby crystal). This is a late game rune.
* Revitalize - Good if you do any significant amount of healing/shielding. This counts life steal, spell vamp, self heals (Warwick's Jaws of the Beast [Q] for example), self shields, and incoming/outgoing shields/heals. It does not count bonus health (elixir of iron), or health regen. Would recommend if build life steal and have a self heal (Warwick, some Nasus, some Xin Zhao), if you have a shield, or if you heal allies. I would not recommend on any champion that builds a single life steal item.
* Second Wind - Generally good. Take this rune if you find the other two unappealing. It's essentially a high gold value health regen. Would suggest treating like a Doran's Shield in that you try to proc it when it's available for a free heal then stay out of champion combat until you can proc it again (i.e. light poking for optimal heals).
* Unsealed Spellbook - Good choice in general, but hard to use. If you are very good at macro-play and can figure out when it is optimal to back and what time it is optimal to have Teleport over Ignite, then try this rune out on a few champions. If you have no idea what I'm talking about, probably don't use this rune. This rune has a high skill floor and ceiling. Riot has plans to make this rune more accessible to the common player, so if you are the common player, probably wait till then to try it. Generally good on champions that require flash to get in range and ult (Annie and Malzahar).
* Glacial Augment - Not quite sure on this one honestly. Seems to be more of a preference choice. I would suggest at least trying it to see if you like it if you think you can use the slow, and if you buy an active item it works with (or could buy it instead of another item). Personally, I like it on Thresh.
* Kleptomancy - Good if you could use Sheen, and if you need a little extra sustain in lane. Gives fair damage, fair sustain (mana and health), and some gold. Got changed up a bit recently so it should be good on champions that didn't buy sheen but still have fairly low cooldowns, as well as not as good on previous users (probably no longer optimal on Ezreal, and probably not OP on Nasus). Try it on some champions who have fairly short cooldowns and need some extra sustain. Make sure that you're comfortable poking the enemy champion often in lane with the champion you chose it on.
* Hextech Flashtraption - Good choice if you are a jungler who want's to be sure to have flash up for dragon/baron pits. Good if you are a champion with an ability that synergizes with flash really well to start an engage (Fiddlesticks flash+ult). Really up to the player almost completely if they want it. Would not suggest getting on most champions or most games. Is a niche rune, and you need to take flash to use.
* Biscuit Delivery - Good on any champion with mana problems. Especially good on any champion that uses Tear, as it gives extra max mana to scale with that. Also gives health regen when eaten. If you eat 2 potions or 2 biscuits it will wait for one to finish before activating the next, if you eat a biscuit and a potion, you will get both health regens at the same time which can be very useful to regain a lot of health when in dire straits. (switching slot with Magical Footwear next patch)
* Perfect Timing - Good pick on most champions. High gold value. Gives you a one use Zhonya's, builds into Guardian Angel, Zhonya's, and Gargoyle Stoneplate. If you are taking Inspiration Primary, and probably if you are taking it secondary, you will want to get it if you plan to build one of those three items or if you think you can end before full build. (Next patch Magical Footwear will be in the same slot, so if you don't want Hextech Flashtraption, you will have to choose between gold advantage from boots or from stopwatch).
* Magical Footwear - Good choice on all champions except Cassiopeia (sorry guys). Has the trade-off of not being able to get boots in laning phase (or most of it if you get some kills/assists) in exchange for free faster boots and a cheaper upgrade. Highly recommended to take on champions you would normally take boots of swiftness on, for example, get Magical Footwear Berserker Grieves Jhin instead of Boots of Swiftness. Same movement speed, plus some damage. The disadvantage may not seem like much for most champions picking this rune, but what if you are up against an Azir mid who bought early boots? You can't buy boots to help out in lane, it will be a hard matchup. (switching slots with Biscuit Delivery next patch, as well as losing the 50 gold less to upgrade).
* Future's Market - Good choice on almost any champions. This rune is really hard to figure out, not in that it's hard to use or has a high skill ceiling, but that it's wording is misleading. It is free gold until you go into debt 8 times in a single game (based on minimum time it would take you to regenerate gold to get out of debt so you can buy again and how much total free gold you would have at that point), at which point you are losing gold. As such you should use it to finish full items, or buy that one item you really need (like boots) just to make lane bearable. The debt portion of it is just there so you can easily see each time you use it. The lending fee is there so you don't get indefinite free gold. To put it simply for anyone who doesn't get what I'm saying, you have 150+5/min extra gold to spend at all times. You lose 50 of _that free gold_ whenever you use this and it doesn't turn on until 2 minutes. That means if you would have to buy 4 items at 2 minutes in to lose gold, and because you can only get one item per debt, that's not possible. By the time you can, it would take 8 items to exceed the free gold. Because you only have 8 item purchases and because of the gold efficiency of full items always being higher than their cost, it is best to only use that free gold to finish full items (although emergency buys are fine too).
* Minion Dematerializer - Gold efficient, although slightly less so than the other two by rough estimates. Gives six free minion kills, and extra damage against the minion type you chose to kill. Suggested uses are on a stacking champion to get stacks faster, on a champion that farms really well to farm-to-fed faster, to help you last hit against melee and cannon minions, and to help you get cannon minions you would have otherwise missed. May also be worth trying to help farm as a melee in top versus a ranged opponent, or as a targon brace user bot side. Really hard to say where it is optimal and whether it's optimal, so try it a bit if you think it could be good and stick to it if you like it.
* Cosmic Insight - Good on most champions. Gives extra CDR on anything that isn't a static cooldown. Suggested to get if you would normally hit 40% CDR and just want a little bit more. Be wary of whether your mana consumption can keep up, the 5% may not seem like a lot in the way of that, but mana consumption versus CDR is not linear.
* Approach Velocity - If you have a slow or hard CC, you should consider to some degree getting this rune no matter the champion. This is in combat movement speed, and a lot of it, albeit directional. In combat movement speed is a lot more valuable than out of combat, not only that, but the person you get it towards will also be slowed at the least. The directional aspect only matters for fights with few enemies and allies nearby (laning phase), late game you may only get movement speed towards an enemy you slowed as Jinx, but you get some towards the stunned Udyr on your team as well.
* Celestial Body - Good on tanks, and few others. Suggested to take on tank supports, and top laners. The 10% less damage may seem like a lot but it actually isn't for most top laners and supports. In both scenarios you aren't the only one dealing damage. Bot lane has the ADC as the highest damage dealer, then minions and supports. Top lane ends up taking a lot of damage from minions during trades due to the range they fight at. So for a support it may lead to only 3% less damage in lane, and for a top laner it might mean 6.5% less damage in lane. And there's always your jungler to help out and skew those numbers more in your favor. A hundred health is a lot, consider that first, then consider whether 6.5% in lane damage for 10 minutes is too high of a price to pay. For Riven it would probably be closer to 9% so not worth it probably, whereas Malphite could care less about 6% less damage, especially since that helps his passive shield which boost his armor and thus his ratios.
Thanks for reading.
Obviously I can't do a TLDR for this since I have to adequately describe each rune, but I hope this is a lot more manageable than the 8+ hours I spent trying to learn this, especially since you can just skip to the tree or runes you care about the most. Might even be able to search key words like support or heal or shield, ADC, etc.