Far Cry is arguably the best singleplayer FPS ever made, in fact it's so good that I bought it again last week and have been playing through again. I run it at 1680x1050x32 max settings (except particle density) 16xAF with HDR and 64bit content on a GTS320, it can still hold its own with pretty much anything graphically.
Gameplay wise what sets it apart from the crowd is that it is both challenging and fun at the same time. Most games that are so unforgiving in terms of enemy AI, long range shooting, harsh terrain etc I would give up on in frustration, but not Far Cry. I can handle being sniped from miles away because I know I can do the same to them, or maybe I don't have to, maybe I can sneak round the side undetected, or just drive straight in with a hummer hosing them down with chaingun and rockets.