I don't think Mass Effect is the gem some people make it out to be, I did enjoy it though, probably won't play it again mind.
Grand Theft Auto 4, really loved Vice City, and I liked San Andreas to a certain extent. However, I just couldn't get into 4. The city was largely empty, the characters weren't massively appealing, and the visuals weren't anything to write home about. Fortunately I found Red Dead Redemption to be a much more enjoyable experience.
Call of Duty, Battlefield, Medal of honor, any of the shooters based around realistic warfare. I just don't like them at all. Admittedly at least CoD 4 seemed like it could be good if you liked that sort of thing, however the others just seemed like clones to me.
God of War, Heavenly Sword, and Bayonetta. Now, I really enjoyed the Devil may cry games, however, I just could never get into any of these. They all seemed mindless hack and slashers. Not to mention I just can't take Bayonetta seriously at all. The demo I played made me cringe with the scenes it had.