You guys almost have to vote for one of hte following:
Starcraft
Warcraft II
Sim City 2000
Command and Conquer/Red Alert original series
Counter-Strike
Half-Life
Doom II
WoW
FF7
I know there are other great games out there, but these were the games that started it all. They are the games we still talk about today, and the ones that we still play today and the ones that have left a legacy.
I agree Halo 2 is awesome, Deus Ex or Doom III or HL2 have been awesome, but they never left the impression that the games above did. I almost have to say CS is the ultimate greatest game ever. It's not the best game to play right now, but considering how many people have played it and still do, you must admit it is the BEST out there