Just like Highlander, there can be only one. I'll get the ball rolling with Grand Theft Auto: San Andreas. Your turn - and don't wuss out and give your Top 10 or any such nonsense. One vote. One game.
Might just have to be Metal Gear Solid - what a spectacular game... Makes me want to dig out my playstation just to play it again...
EverQuest Played some amazing games over the years but nothing even comes close to the revolution that EQ was in the late 90's.
Diabl..............Final Fant............The Legend of.................Tetr...............Crys................ AAAAAAAAAARGH **** YOU!!! (jk ) Damn, this is hard. Has to be Final Fantasy 8, nothing I've ever owned has as much replay as that.
A very odd choice at this moment, Hearts of Iron III. I love me a good strategy game and this is easily the best that I've ever played, got 130hrs on single player and about 40hrs on it's multiplayer
Difficult to say - I would have to say different games for single / multi-player SP - Test Drive Unlimited MP - Half Life 2 Deathmatch
Arma 2 Combined Operations (OK, that's two games so if you are pushing me it would have to be Operation Arrowhead).