Hi,
I guess the reason no-one has replied to your question, is that there are hundreds of possible answers, depending on what you consider a good game.
If you like the first-person shooter, where you run around in a frenzy, picking up weapons, ammo and health and shooting at everything that moves, in order to stay alive longer than your opponent, then the two main games of the moment are:
Unreal Tournament
Quake 3 Arena
If you like a more tactical approach, try something like Half Life Counterstrike.
If you prefer role-playing games, where you view the landscape from a god-like perspective, and your adventures take you through lands of magic and mistery, then try something like
Diablo II
Everquest
Baldur's Gate
If you like running around on some alien planet at the controls of a huge robotic walking machine, completing strategic missions in clan-against-clan fights, try Mechwarrior 3.
Hope this helps a bit...