Yasuo, Lee Sin, Riven
I don't have a problem with any of these champions (but I do dislike Riven when I'm not playing her lol) so I'm going to give unbiased perspective on why people dislike them from my own experiences. Note that I don't play Yasuo and very rarely use Lee/Riven.
In my recent match history, within my last 5 games I went Talon mid against Yasuo. Destroyed him lane going 5/1/0 meaning Yasuo was 0/5/1. In our first team fight (he had Shyv and IE at this point) he picked up a quadra.
From 0/5/1 to 4/5/1 that fast. For the other 99% of champions, you can pretty much ignore them in team fights if they're starved like that. But him having 50% arpen with 100% critical chance still hurts a lot, not to mention the double shield being the thing that stopped him from being instant melted long enough to pull off a quadra.
Lee Sin - when cycling abilities correctly, he very easily and safely secures kills as well as being extremely hard to kill in the right hands.
Riven - chain CC with huge damage and an AD scaling shield all on low CD with no resource to manage makes her way "tankier" than what she should be even with nerf after nerf over the years.
As many posts suggest, it's simply not fun to play against. I personally don't mind, but when in the right hands, they can single handedly change the course of a team fight with their kits.
dive into a fight instead of waiting one sec, ccing them and then watching them explode, riven still has ways of contributing without head on diving but most people don't see riven's split pushing properly in their elo's or flanking well,(i still haven't) Lee completely out competes everyone not named elise in the early game, but does fall off late game since his only way of contributing is an insec, if he is allowed to cycle then your fighting him wrong plain and simple ( he still has an overtuned presence late game because of his kick but thats not a rotation)