I have to go with Half Life 2. It has been the most inovative game I've played in a long time.
I mostly stick with military shooters, but HL2 is a step above the rest.
Playing GR Advanced Warfighter now which is OK.
COD1&2
MOH
The orig Ghost Recon is by far the best squad based game.
ohh, and last,
Max Payne was a lot of fun with bullet time,,,,,,