So I'll keep this short. Yes, we all know the ranking is broken, yes RIOT probably won't look at this post, yes probably they won't address it or fix the issue, but here is another example of how the "Grade ranking" is broken.
Just played a game as jungle Rammus. My score: 3/1/27 with 86 CS
My team's scores:
Vayne 12/3/5 with 191
Caitlyn 19/8/7 with 147
Leona 3/5/20 with 25
Zilean 4/6/15 with 120
Vayne (lowest kill participation and who was rarely there for fights until after 20 min) got an S-. I got an "A."
A fricken "A" despite having the highest KDA by a TON, and the highest kill participation on either team. My CS was certainly not great, but for a jungler whose ganking and racking up a kill or assist every 90 seconds 86 sure ain't bad.
I don't care that much about grade ranks since I get my chests pretty easy each month, but with the new "Mastery" system they actually do contribute to something once you cap out a champ (as my Rammus is).
So it's pretty frustrating to have a great standout performance with 75% kill participation, an almost deathless game, secure all 4 drakes for your team and then be told "Yeah that was somewhat above total crap I guess, but if you want to do good for realz sit up top lane and AFK farm for 30 min k?"