I'm going to have to go with Iron Storm, an alternate history FPS. What if WW1 never ended? It's set in the 60's, built on that premise. Good story line and good graphics for its time. I just replayed it, start to finish, a few months ago, and will probably continue re-installing it from time to time until I can't find an OS to support it.
Flashback - didn't play it on the PC, but I did on the Atari Jaguar (yeah, really), but I never did get to the end. That was a great game too. And I agree on Far Cry - I still pop that in from time to time, but I'll still say Iron Storm is my #1 pick.