I think the worst community I've personally ever come across is the community of people who play fighters online. Now, I'm all right at fighting games. I've never really played one enough to get super familiar with any of the characters, but when I do pick 'em up, I can usually find my way around them. I've only been online with my PS3 for a couple of years, so my experiences are pretty slim, but they're pretty much one and the same.
The first fighter I played online was Soul Calibur IV. I love Soul Calibur. Thought I was pretty decent. Played one round online and lost any desire to do it again. The problem wasn't that I lost: my opponent backed me into a corner and spammed a couple of moves that drained my health like crazy. Wasn't good enough to manage to get around him, be it by jumping over him, counterattacking, blocking, or what have you. He just dropped me. Fine. I can take a loss. What I couldn't take was a solid five minutes of him sending me messages asking whether I was brain-dead or playing with broken fingers. Into the block list he went, never tried SCIV online again.
The second time I tried a fighter online was Street Fighter IV. I was Blanka, playing against Ken. Again, ass kicked. Fine. Again, a string of messages asking me if I'd ever played a video game before. Again, asshole goes on the block list. Just because I lost, doesn't mean you're unquestionably superior to me. In fact, that game was a helluva lot more even than my SCIV one. I actually thought I'd done pretty well. But, again, after that, I lost all desire to play online.
The third time I played a fighter online was Marvel vs. Capcom 3. I'd never played an MvC game before, so I figured I'd give it a shot. Didn't do too hot in the arcade mode (got my ass handed to me by the comp, to be honest), but I thought I'd give the online a whirl anyway. Get into a match, and almost immediately start getting my ass kicked. Middle of the match, my connection drops. Out of the game I go. Thirty seconds later, here comes the most shocking message I've ever read in my life. The gist of it was that I was a pansy-ass ****** who quit because I couldn't handle a loss like a real man because I liked it up the bum. Apparently, being gay and being a sore loser are co-dependent. Didn't know that.
The last time I played a fighter online was this past September. I picked up Persona 4: Arena, mostly because I love the Persona games. Did really well against the computer, so I thought I'd try playing it online. Teddie was my fighter of choice, and I played four matches. Won all four. I was pretty proud of myself. After the last match, the guy started spamming me the same message, over and over. It was simply 'learn some moves.' Just out of curiosity, I watched the match back. I had a pretty well-rounded set of moves going. Didn't know what the hell he was talking about. Then I noticed that every single move he hit me with was one of three exact same moves. This irked me a little bit, so I sent him back 'I don't need them to beat you, obviously. Grow the fuck up,' and then added him to my block list. Don't have any desire to try a fighter online again, at this point.
But, really, just about any game you play online is going to have a terrible community. The most vocal members of any online game are the ones you don't want to hear from. The jackasses spamming racist, sexist, trolling remarks over trade chat in a capital city in World of Warcraft are a minority, but they talk so loud they seem to be overwhelming. The assholes who shriek into their mics in Call of Duty are, I've found, actually significantly less than those who are using them mics to try and coordinate with each other. They're just a hell of a lot louder. I tend to mute everyone in lobbies, anyway. I only play online when my friends are on, and we just use Skype to chat with each other. I hate the internet for that reason: I know there are thoughtful, intelligent people around, but they're buried under an overtly hateful layer of super-loud assholes.