I'd probably have to say The Conduit. This game really had potential, but to me it was just terrible. Sure, it could do some nice things with the engine, but I thought it still looked bad. The story was crappy in that it was probably already used before. And the multiplayer really wasn't as good as it could of been to me.
 
Actually, maybe it's not that it was the worst game I've played this year, but the fact that it disappointed me the most.