My Fave single-player FPS campaigns:
1) Stalker SoC (w/Complete Mod)
2) HL2 + Episodes
3) FEAR
4) Prey
5) Stalker CoP (w/Complete Mod)
Some other good ones are: Borderlands (though it's most fun w/friends online doing Co-op), Singularity, Metro, CoD MW, CoD BO.