Operation Flashpoint was one of my favorite FPS's. It is very realistic though, so when you get shot, you are either crippled to the point where you will bleed to death in a few minutes, or you die instantly. The enemy AI is generally retarded as well. But the game itself has great atmosphere, and is a lot of challenge.
It is pretty much a war simulator, and it does that pretty well. I wish that I didn't lose the CD's a few years ago, because I have been wanting to play it again.