Modern Warfare 2, closely followed by Black Ops. In terms of bad full price purchases that is. Worst game I've purchased ever, ignoring price? Frontlines: Fuel of War. That was something under 5 bucks on Steam... and I wish I had that money back. I regret all 73 minutes of that game I played.
As for MW2 being barely able to be worse than BO... the inner part of the disc broke, so now the 360 won't read it. Not to mention the only part of the game worth playing was Spec Ops, and that's not worth 60 bucks. Not to mention BO having only a bit of fun with Combat Training and the zombies. But then again, if I really want to play a zombie game, I go play Killing Floor. I really should have saved all that money... especially considering to even get BO, I had to spend in excess of 300 bucks since around that time my 360 died, so I spend 200 getting a new one, and also 2 1-yr XBL thingies, one for me and one for a friend. I also regret those purchases... I don't play online much anyways. I get bored easily with people online.