Halo 3 was and is an amazing game. CoD4 was good and theres no argument the online multiplayer was slightly better but crank the difficulty up to legendary on Halo 3 (add a few skulls if your a sadist) no fps can touch the awesome ai that makes the campaign mode so cool, especially with a mate.

P.S. if your looking for a mind blowing story from an fps you probably won't find it here. Shooters are supposed to be fun and this doesn't disappoint.