I haven't come across the worst graphics in an Xbox game just yet. I think that the worst I've come across is Tetris Worlds, which is not only a waste of the Xbox's abillities, but nothing was done to the game to make it look presentable. It's so stale.
I was disappointed by the visuals in Amped. I thought there was going to be a good level of bump-mapping and lighting effects, since it was an Xbox launch title, and you would have thought the developers would have added a little more pop to the game. But it looks very plain, compared to the vibrant and lively SSX series.
As for what I've heard what the worst looking Xbox titles were, I think Pulse Racer, Stake, Smashing Drive and Aquaman were said to look like ass.