Well, I can't say Burnout Paradise, since that's not a shooter...
I'm gonna have to go with Left 4 Dead, back when I played it last year. I loved a majority of the people I played with. Lots of people talking, cooperating, and it was just fun.
And I have no idea what you guys are saying about TF2. On the 360, too many teams either put too much focus on attacking or defending, not enough people have microphones, hosts abuse the ability to put limits on the number of classes, stuff like that. I love the game to death, but the people that play it... ugh.
And please, oh please, don't bash me for playing on consoles.