Rushers. Campers are fine, and I camp myself, usually because I prefer to take a cautious or sniping role in FPSs. But rushers? On any FPS they just charge in and don't bother playing by any tactics, which makes them easier to kill, but it infuriates me that people are so stupid. Also, when my teammates rush, they die and I'm left on my own in the middle of hostile territory on the battlefield. And in any RTS, rushers are people who literally create a massive army in the first minute or so (usually with cheats) and then Zerg Rush you when you're still building a solid defence. Something that in a real life situation no commander would try because it would be suicide. And then the game ends with you losing in a few minutes before you get the chance to actually play a decent game, and you leave without having any sort of fun.
Sorry to rant like that. But yeah, I hate rushers the most.