Final Fantasy 13 easily.
I tried to like it, I really did - but between the completely linear levels, terribly told story (the story by itself could be decent but the way its presented is horribly paced, and the way the characters act is completely idiotic, on top of the replacement of character's emotional development with stupid melodrama), and an auto-pilot battle system (yeah, yeah, I know paradigm shifts - but even at its best I still felt like I was pretty much on auto-pilot for most the game).
I don't think I've ever regretted buying and investing so much time in a game before.