As has been mentioned, the pod-racers and droidekas had good designs. I don't understand those who hate the pods because they're "obvious copies of chariots." As if everything in the OT was completely separate from real world concepts.

I also like Theed, Otah Gunga (or whatever that Gungan city was called). The Gungans...not so much, though they could have been tolerable if the ones that got screen time weren't so goofy. General Grievous's wheel thing isn't bad, although I don't know if one could really consider it a success in terms of design.