Well I'll exclude games in genres I dislike as that's a bit unfair. Football games, etc.
I'm quite lucky in the fact I generally buy games I know I'll like. I can't say I've played a horrible game.
I could say Zelda Windwaker, But that's due to me finding the massive open areas you had to sail across tedious\boring. To the point I stopped playing. But I wouldn't mind actually going back to it and giving it another chance. I mean it's a zelda game!!
I can definitely say I hated Superman 64