You are NOT good at...
Yasuo top.
I see it in the lobby, I make my comment "oh yas top, am I in brunz again?" Then they feed, like every yas top I ever see in any game, they feed. There was a time when Yasuo in the top lane was really good, and he can still womp some top laners... but more or less that time has passed and only the derp among us troll their team with it. The only people playing Yasuo top are smurfs picking on lower skill players, and people who want to be carried. You arent good at Yasuo top.
Teemo.
Teemo is a good champ that can be an incredible lane bully. That is if you know your match up. If you pick Teemo before the enemy top laner is determined, and lets face it you do because youre not good at him, then you will feed. Several times I see people who are first pick on our team pick Teemo, then wonder how they got so unlucky to be matched up against Irelia. Theyre really shocked. You arent good at Teemo.
Lucian.
A while back Riot buffed luc, and before that, I told people to "pls dont pick luc hes weak" and they would anyway. Then Riot buffed him and in the patch notes they pretty much called him a troll pick... I felt vindicated for telling all those people to not pick Lucian. Some luc buffs, BotRK & Black Cleaver changes later and luc is META again... now we have tons of imbeciles flavor of the monthing a mechanically complicated champion they suck at cause "hurr durr OP freelo" making them feed and blame supports everywhere. You arent good at Lucian.
Special mentions, Master Yi & Vayne
I refer to Vayne, Yi, and Yasuo as the "Bronze trifecta of fail." I dont know why exactly, but the worst players in the game are attracted to these champs. I think it has something to do with them being absolutely destroyed a few times by them and they think "well I must master this power that has undone me!" They dont realize they were destroyed by these champs because theyre bad and it just happened to look "cool" when those 3 did it. Every Bronze game I played to leave that rank this season I dreaded seeing those champs because I knew each one would tumble, dash, or alpha into a fight they couldnt win and feed then tilt and blame everyone else every time. You arent good at the Bronze trifecta of fail.
Ok, just a bit of a rant to vent my frustrations for seeing the baddie magnet champs feed over and over.
Do you guys have your own list of "pls dont play" champs you hate seeing people pick in your lobbies?