The worst RPG I've played? Werewolf: the Forsaken. Bad Storyteller, no biscuit. You do not saddle the party with three DMPCs - one for each of the three PC's schticks but better. And then deliberately quashing all the PC plans ever so the NPCs could do it all.
The worst I've tried playing - I can't remember its name. But when it takes 20 minutes to resolve where the first bullet goes (right down to how much wall it penetrated to work out if it hit) we abandoned any attempt at a playtest. There are a lot of RPGs out there that are simply non-functional.
The worst I've had fun playing: Middle Earth Role Playing. MERP is a decent gritty swords and sorcery system (it's essentially Rolemaster Lite - something that vastly improves Chartmaster). It's just it has critical tables with results like "Skeleton liquified. Use a spatula." Nothing wrong with that sort of black humour - but it isn't Tolkein.
That would be Dread - there's a very sub-par game of it on Tabletop, and I strongly recommend it for horror one-shots. It's the tensest horror game I know.
You loathe a game about playing teenagers that encourages peer pressure to be effective? While being very careful to make sure that you never have to bow to it if you decide not to? To me that sort of stuff makes it much more immediate and immersive and aligns my thought processes far more with the character.
And it really is never "Your character wouldn't do this". It's always "There's a cookie in it for showing the side of you others think is the most interesting or doing what they want". Also not every system avoids using social mechanics against other PCs (I can think of a lot of RPGs with no such hard restriction). It's simply that most RPGs with social mechanics make them god mode abilities that are no fun to be on the receiving end of and then patch the system by saying "We know this makes little sense - so you don't do it to other PCs".