I'm torn between two titles, so I'll just name them both.
Haze made my eyes and ears bleed. It offended my very soul, it was so horrible. Yes, it was a relatively early foray into this generation of console gaming, but that doesn't justify having graphics that could've been pulled off last generation, or even the generation before. Add to that the horrendous level design, the atrocious voice acting and just the overall blandness of the gameplay and I can't help but name it one of the worst I've ever played.
Terminator: Salvation is the other, and what a horrible game that was. I picked it up on release day, went home and sat down to play it co-op with a buddy of mine. Two hours later, we'd finished the game and had no idea what to do with the rest of our day. The balancing of the AI was terrible, being extremely difficult in some sections (I'm looking at you, vehicle sections) and retardedly easy in others. The story was pretty lame, adn the voice acting was, once again, pretty abysmal. The best part of the voice acting was Common in the same role from the movie of the same name, and he's not that great to begin with (though he was decent in the game and movie). Really, really bad game.