MMO: WoW. Bonus: Action-MMO: TERA.
ARPG: Diablo. (Luckily the devs of PoE capitalized on this opportunity)
Every game that monopolized a genre is doing this now. And yet, everyone's trying to be the next big shiny "NEW" thing instead of doing what PoE did and make a good spiritual successor.
Every 'classic' MMO is trying to be 'player driven', 'sandbox', 'no grind', etc and they've pretty much all 'died' with the exception of FF14, ESO, and GW2. Why are devs not doing what's shown to work but simply with a better managed game? Most other 'successful' MMOs were terribly managed and ended up dying because of it. Aion, TERA, etc for example. People swarm to every new MMO in the hopes of finding something even decent, and they're always bogged down by horrible things that were done in the attempt to be 'unique'. The only game that looks to be aiming to take over WoW's dying legacy at this point is "Pantheon: Rise of the Fallen". FF14, while successful and pretty well-managed, is a purely PvE game (until recently with their 'for fun' PvP modes). While that appeals to some, it gets stale very fast if the content released doesn't keep up with the playerbase, which was a large part of the reason why TERA ended up failing; rushed content release while also basically killing PvP content entirely. Luckily FF14 hasn't fallen to this yet, but that leaves many players unsatisfied for many months at a time between content releases. There's nothing outside of the effectively ancient ArcheAge: Unchained for PvP, and that hype is dying down very fast. Hopefully Pantheon is good after the next 3 years of waiting for its release. There's always the fabled "Project EXA" (TERA2) vaporware too.
Diablo 3, while driving itself into a hole even if it recovered, never grew to what it could have been, and PoE pretty much took its entire playerbase outside of the most hardcore fans because of this. Nobody can deny that PoE is simply a better game overall, and that the devs care much more for it. While Diablo 4 was announced, so was Path of Exile 2. Its very likely Diablo massively falters in popularity with its next release unless the devs really pour their heart and souls into it, which is unlikely to do as much as they'd wish because they will have much less creative freedom working under a massive, now uninspired, company like Blizzard.
And then there's LoL. Effectively committing suicide for the past few years, similar to WoW, but with no one to truly take its place. Its very likely that DotA2 will remain its biggest competition even with all of the cash grab MOBAs being tossed to the wind to see what sails. None of them feel inspired. None of them give a reason to play them over the familiarity of LoL or DotA, depending on your preference. Its unlikely that a game puts as much creative effort into their champions as either (even if from a gameplay design/balance standpoint LoL does horribly), and that's going to be one of the major factors for drawing peoples attention from either. There's always a chance that LoL starts to go back to its roots as Riot (and possibly Tencent) rely less on LoL alone with the recent game announcements, but I'm going to remain extremely skeptical...and it'd be several years down the line if it were to happen anyway.
What do you think of the state gaming in the past few years compared to the golden age of games of each genre?