I play Rainbow Six Vegas online most of the time (its about time we had some more download content, new weapons etc)
I used to play a lot of COD3, but it got very boring after a while, far too many planks team killing and no team play.
Tried Gears of War, but really could not get along with it. I also tried GRAW2 and I didn't like the way it played either, not a fan of 3rd person and the FPS view was awful. There seemed to be a lot of lag which killed the gameplay.
Battlefront was excellent and my first online experience with the 360, didn't think there were that many people playing it now.