My most enjoyable FPS is probably CS:S simply because of how many different game-modes the community has come up with over the years. Stuff like Jailbreak, Surf, Hosties, Minigames, Death run, etc. I can't think of any other game that has such a broad appeal.

I will say Borderlands 2 for the perfect mix between FPS and RPG.

For me is the original Doom. Still replay it from time to time :)

I'm gonna give an honorable mention to Halo because that's the first true FPS I ever played.

Duke Nukem 3D

I was very fond of System Shock back in the day

First Far Cry.

Thanks a bunch! ^_^

Some of my favorite FPS games are Doom, Duke Nukem, and Serious Sam.