The worst game I have ever played would have to be Hell's Kitchen, on both PC and on the DS. It is horrible. Not fun at all and just awful. Words cannot describe it really.
I'm not sure what the name was but a few years back I played a bowling game on the Game Cube and it was just so horrible, I've never played a game that horrible before.
I think those who owned a Nintendo 64 can all agree that Superman 64 is the worst game in creation. I played it when I was 4 and was still bored to death.