Call of Duty, for well, the obvious reasons. The fans are predominantly mindless twelve year olds that I can't fathom the fact that anything outside of their beloved series is fun. Nevermind the fact that they're all racist, homophobic, immature BLAH, BLAH, BLAH, WORDS. You can see what I'm saying.
Also, a lot of fans of the Elder Scrolls series, for a few reasons. The first of which is that so many of them have never even played earlier titles (that is, pre-Oblivion) and generally refuse to, due to the fact that the graphics are dated or some equally stupid excuse. "What, you mean my skills determine my chance to hit? Fuck that!" Yeah. Nevermind the fact that the community is divided between the mindless lovers of Morrowind and Oblivion, who constantly argue with one another (or at least did so until everyone started wanking over Skyrim) about really stupid crap. 5% of the community is awesome, the other 95% of the community is crappy.