Or if you have friends in real life, not just the Internet. Halo 3 >>>>> Nearly every other FPS because you can have a couple friends over and all--get this--jump on Live together. You can like, call up your friend, and instead of him saying, "Sorry, I'm playing Halo tonight, forget it," he can say, "Yeah, I'm playing Halo tonight, come on over." It's freaking awesome, and the fact that most games don't do this show how ignorant developers are of social dynamics.
Agreed. Other games come and go in my circle of friends but it always comes back to Halo for that reason.