It's got to be Hey Ma! Honestly, I never tire of that album. My iTunes "plays" counts for each of the tracks is in the hundreds. "Upside Down" has 444 listens alone! Funny that, I would have though that "Of Monsters, Heroes And Men" or "Waterfall" would have been the songs that I listened to most.
Millionaires and Pleased to Meet You would be the other choices. I don't think they've left my car's CD changer in at least a year.