Unreal Tournament (the original).
Psychedelic colors, sci-fi settings, great (fantastic/unrealistic) gameplay, bite sized time requirements, amazing soundtrack -- My biggest FPS benchmark.
FarCry is probably the second; the tropical setting was just too cool. Crysis tried to emulate but took itself too seriously and toned up the military aspect too much imo.
-Edit-
Reading posts I've come across a lot of other good ones.
Mirror's Edge - not what I'd call a fpS, but a revolutionary game nonetheless.
Bethesda - again, not straight fps, but great stuff.
No One Lives Forever (1/2) - I played these way past when they came out, but the fact that I still had so much fun just goes to show how great these games are.
TimeSplitters (1/2) - Very similar to UT, and both single player and multiplayer were amazing.