I would have to say Grand Theft Auto IV, because back during release, I was so hyped for the game, and had even gotten during launch day, but it was not what I was expecting. I thought it was going to be another crazy adventure like San Andreas was (lucky Saints Row exists for that purpose), but instead it was "grim, dark, and realistic" and back then, not only did I not care to play a game like that, but I didn't really understand the story of it either.
I had shelved the game since 2008, not once looking back at it to complete it, however earlier this year, I got the urge to go through it again, so I had started a new game, and I had found myself really liking the story. I would have to say that Grand Theft Auto IV went from being a game that I had totally hated to being one of my favorite games of all time.