Mos Espa Arena. I loved that level for the hidden areas, like that Sandcrawler at the start with the Jawas in it, and inside the arena, the structures covered in the shadow. There's just something about that atmosphere that I really love.
The trading in Mos Espa was another aspect I really loved, but I hated it when some of the things bugged out; like that Matt Rags guy, sometimes you couldn't even start a conversation with him, and Barbo; I hated that guy when he wouldn't let me in the back :P
It's hard to choose the worst level, all have their own charm, but if I had to pick one it has to be Coruscant, that puzzle in the suburbs drove me crazy sometimes.