Definitely 360 gamers. Hands down.
As someone who plays on just about every system imaginable whenever I get the chance, 360 gamers are usually in one of these two groups of personalities:
1) Totally fine, calm, fun to play with, occasional good-natured smack-talk, but otherwise perfectly fine.
2) Viciously aggressive, gung-ho, nobody's better than me, if you're not the best gamer on earth fuck off, constant loud irritating offensive vulgar smack-talk, less than no sense of team-work or camaraderie, plain out dicks.
While playing a game of Halo: Reach with my brother some time ago, we entered a Team-Deathmatch and started our usual team-play tactics by flanking anyone who grabbed a vehicle to run us down, and blowing them sky-high, or hijacking it.
The host got pissed at this [he was on OUR team, by the way] so he rocketed the Specter we'd hijacked just to kill us. That's fine, I'm cool with that. I ignored it.
After that, I got tagged by a sniper, so I grabbed a shotgun and a battle rifle and went hunting. When I found him, he started hopping around like a retard trying to head-shot me, and I opened up with my battle rifle. Evidently, the host took offense to this, so he SHOT ME FROM BEHIND, then KICKED ME.
So yeah, that basically sums up almost all of my online interactions with any 360 gamer who falls into the second category, and pretty much cements their position as the most angry gamer-group on the planet, because cat2 describes about 99% of every 360 gamer I've ever met.
Exceptions are: 1) An awesome dude in Black-Ops who was humming banana phone the whole game, while slaughtering everybody in his sights.
2) My entire team in another Black-Ops match who set up a massive sniper-death-zone in Array instead of run-and-gun-then-die-and-smack-talk-for-five-minutes-afterwards-in-the-lobby.
3) A chick in L4D2 who shot-gunned the chauvinist-prick right out of two douches on our team by saving their asses from tanks and hunters every two minutes, and had a sense of humor to boot.
4) My Brother Alex, who plays video-games to have fun, instead of trying to prove he's 'the best'
5)...Nope...that's about it.