Depends on whay type of FPS (Single or Multi)
Single player, hmm has to be Deus EX or Far Cry.
Multiplayer, hard choice, I've played loads to death but I think it would be in this order -
Joint operations (and add on)
Call of Duty
Return to Castel Wolfenstein
BF:V
BF1942
Raven Shield
Quake 2 Rocket Arena
Maybe some others