I think the ipod and to a lesser extent the ds, are both better design wise, but psp has a more expensive look to it.
If we are talking exclusively about design though, I would say: ipod>sp>>ds>psp
As many people have said, the ipods simplicity may not be possible on a non clamshell handheld, but the psp is definitely too cluttered (too many buttons, text, and logos).
The DS is a cool design but it just doesnt compete with the sp because the edges arent flush with each other when the unit is closed, making the two parts look seperate.
Woulda been cool if the top screen of the ds was widescreen I think, but I guess the speakers take up that space.