Saint's Row 2, I can goof off in Stilwater for hours and accomplish nothing.
Next is Fallout 3 because I can plough through every inch of map looking for every last item in every last power station, activate every single radio tower, find all the little nooks the Communists were hiding in to spy on the US etc., and never get tired of it.
Crackdown was ok once you build up your skills, but I got bored with it pretty fast. I could only really enjoy it in short bursts. GTAIV I played purely for missions/story, I didn't feel compelled to mess around like I had in previous GTA games.