A Game With Good Online Play?

I'm about to get live and I'm going to get a new game to play it with, I want one with a large community that doesnt seem to be dwindling any time soon... They had that GeOW tournament recently.. is the community for that pretty large? Basically, I'm looking for suggestions

-Thanks!

Rainbow Six Vegas

Also

Xbox 360 Top Live Titles (UU's)
1 Gears of War
2 Tom Clancy's Rainbow Six Vegas
3 Guitar Hero II
4 Call Of Duty 3
5 GRAW 2
6 Boom Boom Rocket
7 Crackdown
8 Oblivion
9 Pro Evolution Soccer 6
10 Madden NFL 07

Well since everyone is saying Rainbow Six and Graw 2, I'm gonna go out on a limb and say Crackdown and Chromehounds.