As for games that surprised me:
Bayonetta: I even made a thread about how shitty it looked. Became one of my favorite games of all time.
Gears of War: Didn't even really want this game. Looked generic and dull. I only bought it because it was on sale and I was out of desirable Xbox 360 games. Loved it.
Forza Horizon: I'm not a fan of realistic racing games but it was like $5. Was hardly realistic (I guess) but addictive as heck! Top quality experience.