1) Duke Nukem 3D
2) Half Life
3) Quake
My reasons :
Duke is just such a fun game to play, i love a sense of humour in a game, and of all the fps games, this is the one i still play.
Half Life has so many cool mods, was never a big cs fan, but i loved team fortress and deathmatch classic
Quake is probably the fps i've played the most, when i was the helpdesk manager for GT Interactive in the UK, i had plenty of reasons to play this game, seeing as we supported the game. Imagine an office with 8 people who would boot up their computers in the morning, and load quake before the important things like their email and helpdesk software !