Not played mine for ages as the games I have are not 'child friendly' (i.e. swearing and killing stuff) so have to be aware of where my 5 year old is at all times.
Loved GTA IV for the ability to do pretty much whatever you wanted as there was no real 'fixed script' of how to play the game.
I also loved Rainbow Six Vegas 2 - another FPS that kept me occupied for ages with its missions. The ability to 'guide' and instruct your accomplices (controlled by the computer but going where you point them, and undertaking actions you specify etc.) was a novelty (well, for me at least, not really used to gaming) and an interesting twist.
Call of Duty 4 was pretty good, but over far too quickly, and seemed very easy for the most part.
I have Haze which I haven't played yet, and Resistance: Fall of Man which is utter crap. I also have GT 5: Prologue which (IMO) is dull and far too repetative... but then I never really was one for driving simulator type games.