Fable1/2 - very good if you like European type fantasy, very enjoyable games, with in my opinion some of the nicest settings you will ever dream about building fortresses of doom in...I mean it's pretty...
Crackdown - Very fun, and the combat/levelling system is enjoyable enough that I can play and replay the game.
Fallout 3 - There's a reason the majority of Rpg/whatever fans like it, find out.
Mercenaries 2 - Lots of fun, I've finished it three times, can't really explain why, it's just that enjoyable.
KOTOR1/2 - If you haven't played it you better have never had a computer or xbox, or you should punch yourself in the face.
Thief 3 - Fun thieving system, with enjoyable stealth gameplay.
I've seriously played them all for something like a combined 400 hours, though I still haven't finished Thief 3 (if you have played it, then you understand, I can't go through the cradle again...I know, sad and un-manly but they scare the shit out of me).