GTA4.
I loved it when I first started playing it because it was really nice looking and I had yet to unlock all the goodies. By the time I got to 100% (yes 100) and realized that the only goodie was a crappy attack chopper that respawns nowhere near any of the safehouses, my opinion had changed drastically.
No More Heroes had a really fun tutorial level but unfortunately goes downhill fast from there. I thought I was going to love it but it was the only game I found myself selling last year. The grass cutting minigame was the point of no return- from sucking.