in no particular order based purely on enjoyment (and addiction):
Battlefield 1942 - Wake Island demo says it all really.
Deus Ex - Whats not to like about Deus Ex!
Quake 2 - SP & MP, I was a late starter when it came to quake
Quake 3 Areana - Multiplayer was the shit.
Team Foretress 2 - MEDEEEECC!!!
Leisure Suit Larry in the land of the lounge Lizards - Ken Sent me. First game i ever played on a PC (286)
Leisure Suit Larry 2 - getting off that damn cruise ship made my brain hurt.
Mafia - probably the best single player game ive ever played due to a great story. Good times.
Tomb Raider - the original on PSX. Mmmm Lara. Mmmm pixelated boobies.
Modern Warfare 2 - I know i Know, but I never played the original Modern Warfare so i really enjoy(ed) this