Halo 3, Call of Duty 3,4, and World at War. Halo 2 is also pretty good. To me halo 3 and Call of Duty World at War are the best for online out of their series.
Fallout 3's a great game but as everyone else has pointed out Halo 3 has to be the best. There's no other game out with such great multiplayer as the Halo series. Call of Duty is great as well I treat it a break from halo for a while as I check out the different game play style.
fable 2 hands down, The hugeness of the game is still what boggles me, I also enjoy the idea of choosing your own fate.
the gunner idea is a fresh one from the original fable (crossbow man), though it is kind of frustrating to die and then be revived with scars from the horrific battle.
