Half-Life: Counter-Strike
Medal of Honor Allied Assault
Diablo II
EverQuest
Half-Life: Day of Defeat
Age of Empires II
Starcraft
Unreal Tournament
Command & Conquer: Renegade
Asheron's Call

I would say Quake 3 but no one plays it much anymore, sadly.

You should have Wolfenstein and Ultima Online as well.