Damn so many I cant even remember most but heres some of them from back in the day to present...
Diablo I - I kicked many ass on battle.net
Subspace - Another multiplayer I owned in
Quake I - Yea another multiplayer
Quake III - Multiplayer
Unreal Tournament - Multiplayer
Ultima Online - Another multi player
Everquest - Another multi player
Medal of Honor - Multiplayer
Wolfenstein and ET - Both multiplayer
Damn I have no life so im not even gonna mention the console games...