I was going to say Halo ODST, which totally should have cost $30, but then I remembered the REAL worst game I've ever bought at full price:
FUEL.
An open-world racing game with monster trucks, motorcycles, extreme weather, some explosions, and multiplayer? How the hell could that not be awesome? I overlooked the somewhat negative previews of the game and pre-ordered TWO COPIES of it so I could play with my dad and brother.
It's a completely barren, boring world with like 50 bland races spread out over the whole massive thing, and the racing is worse than a WoW vehicle quest. I traded them both in for a quarter of the price I bought them for within a week, after playing it for about 2 hours.
That game is why I always go with my instincts (and reviews when applicable), and I only buy high-quality mainstream games on release. Everything else requires a demo or a price drop.