GTA IV.
Was bored silly by the first six hours, but the campaign ended strongly and the two DLCs were both excellent.
But I don't think it's anywhere near the mammoth achievement critics and a lot of gamers hold it out to be. Cover system that often didn't work right, auto-aim system that made combat far too easy, crappy vehicle controls, that fucking friendship system, and all those completely pointless, boring missions in the first six hours (do we really need a mission where we learn to throw a brick or learn how to fist fight... neither are ever required to be used later in the game) are all are pretty major marks against it.