For me it's Black Ops which I think is the best FPS ever. There I admit it. What about you guys

S.T.A.L.K.E.R , a lot of times i forget that i am playing and just "living" it, i play it on hardest mode, and the stealth mechanics and the fact that you use everything you got from equipment for survival  is probably what made me play it a lot (example try to use the binoculars to plan your ambush,or item harvesting)

I've never heard of that game when did it come out?

Duke Nukem 3D, fifteen years ago. The only FPS I ever finished.

Most likely Left 4 Dead 2 or Killzone 2. I'm not sure which I've played more. I've also spent a lot of time in Borderlands, however that's not strictly a FPS. There hasn't that many intersting FPS since Bad Company 2 IMO and there aren't any I'm really looking forward to outside of maybe Battlefield 3.

it's either Goldeneye 007 or Tactical Ops: Assault on Terror, and I believe it's the second. since I started to measure it with Xfire (September 2004), it's been 1.245 hours of game time, but I played it in 2002 and 2003 too.

edit: if it's in a single session, then I really don't know, I usually don't spend a LOT of time playing a single game.

Call of Duty: Modern Warfare Reflex
First Call of Duty game I ever got into. If it wasn't for lag shooting then I'd still be playing it over Black Ops.

Whatever it took me for one single play through in Prime 3.
I did spend a whole day playing MW2 in my cousin's house in U.S.A, it was actually good

its a PC only game so thats why you haven't heard of it, i don't remember exactly when it was released though, but the third game was released in 2010

Wolfenstein 3D, one of the best FPS ever made.