I would have to say Halo 1, but I loved that game, So I'll let it go for now, but recently: Halo Reach.
I was doing campaign on legendary solo, and I'm not that good at the game, so I tried to do things stealthily while my teammates would just kinda stay behind and run into walls. However, Jun didn't want to run into walls one day. Instead he decided to kill me.
I was sneaking up behind an elite... elite. You know, the ones that are golden? Generals I guess. Anyway, I was about to assassinate him when suddenly out of nowhere I hear Jun scream, "OPEN FIRE!" and start shooting wildly at the elite. Then, instead of turning towards the five shots that were just fired at him, he turns around and stares directly at me for about half a second before punting my useless spartan body off a nearby cliff.
I think I hate Jun worse than Kat, and she drove me off a cliff 15 times before I decided to drive.