Well Halo 3 is great for those who like sticking together more then breaking apart
CoD is great for those who like to lone wolf that shit and don't mind taking one for the team
Gears 2 is HIGHLY based around sticking together and along with great guns and things to do it for the band of brothers kinda guy
I would have to say that my most enjoyable online experience to date would have to be Godfather 2 online
along with guns blazing and bombs flying you can completely tell your men what to do in fast paced crazy action.
Hope this help