League of Legends is pretty high up there for me. It's a team game and there's a lot of people who are both toxic and don't want to work as a team, which comes off as odd to me... because it's a team game.
Maplestory was pretty bad for me. I leveled to 30 and between then and the start, I ran into a LOT of kill-stealers and people who'd spam chats for no real reason.
World of Warcraft is a close third, though I wouldn't say it was as bad as the previous two above.
osu! I've found has had a really friendly community. I'm on the forums all the time (I've posted over 1000 times, I'm not sure why my counter reset recently) and I've rarely had to deal with people badmouthing others in a really hardcore way. I've asked a lot of questions and brought up some other topics and I generally get friendly answers.