I am going to have to go with Halo 2. Although Half-Life 2's story is one of the best I have ever seen not to mention the graphics, the overall fun level of Halo 2 online is unparalleled by any other game.
PS> My Gamertag is x 1337Spartan x if anyone with xboxlive wants to look me up.
the X-Box versions.........PC version of Halo is just kicka$$, and the CE edition of it is a free download, as long as you got a valid key
Halflife two isnt bad......
Half-Life 2 is propbably the best Single Player FPS in the past decade. And, well, you cannot compete against it. B)
You cannot vote based on just Single Player, I am talking overall.