Well, not exactly a shooter per se, and certainly not FPS, but Vice City and the sequels are a lot of fun, I spent days on Sims and Roller Coaster Tycoon, but those are builders, not shooters. Civ and Empire Earth are great also, but also they are less shooters. Well.... hmmmm......
Doom is old, but lots of fun, still, and like the poster above i think i died like a million times in the hallways in Doom2.
Remember when you left your bodies behind? Haha, mine littered the hallways and a friend said "Whats that?" And Sam said, "Oh, thats Teal, there... and there... and there..." Haha, but still it was great fun, even if it was 3 or 4 seconds at a time, die and reload the level.