I played Darkwatch through on Hard the other day and beat it..
Now I'm playing Crimson Skies: High Road to Revenge and Chronicles of Riddick: Escape from Butcher Bay, both on original Xbox.
Crimson Skies has always been a classic, and VERY fun game to play, so I'm enjoying it.
Riddick.. This is a very unique FPS/action-adventure game. It's not just your typical run & gun-typed shooter. It has a very natural, organic feel when your character walks. Heavily relies on stealth tactics, just as much as blasting enemies. I'm almost sure it uses the Doom 3 engine as well. It looks very similar.
It also has one of the best hand-to-hand combat systems I've ever used in a FPS game before. You pull the right-trigger to punch, but if you move the left thumb stick up/down/left/right, while pulling the right-trigger, you can do different punches and turn them into combos.
This game is very good, and now I see why IGN rated it 9.3