Probably Driver 3.
As a big fan of Driver, I had been following the game for years, and it showed a great deal of promise based on what had been said and shown. It was doing a lot of cool stuff with physics, it boasted like 80 vehicles, and it had third person combat. So I pre-ordered it, brought it home, and became almost instantly disappointed. The framerate made the game unplayable. Everyone had been bashing the gameplay, and it's true that the on-foot stuff was poorly done, but I found it literally unplayable just on a technical level. I could have forgiven it otherwise, and that is why it was so disappointing.
Thankfully I was able to get a refund. Blockbuster for some reason charged me $80 for the game.