I've heard that the E.T. game for Atari was the worst ever made, but I've never played it. It was so bad, Atari buried all the unsold cartridges in a landfill in New Mexico. I'm totally serious.
But as for worst I've played, one of them would be Blinx. I don't know if any of you've heard of it, but it was terrible. You're a cat-janitor, who uses his magic vaccuum to suck up tables and shoot them at enemies. But if you get enough crystals, you can alter time. Trust me, it wasn't worth my time.
Then The Hobbit for the GameCube. The opening cutscene looks pretty cool, but after that the animation fails and you're set to do menial labor for the townspeople. I think in the first hour of gameplay, all you can do is jump, with the occasional pick-up-the-stick-and-swing-it-around.
And Finding Nemo was pretty terrible. I really hated it. Swimming through rings of bubbles and following other fish isn't really my idea of fun. I quit after level 7.