To tell the truth, there aren't any levels, that I don't like. But Gardens of Theed is actually the one I admire the most. I remember playing it for the first time - and it was there where I had this interesting felling, that Star Wars is not just Empire with grey-to-black walls, as well as a ruined "perfectly good city" of Barons Hed, but it is also a well-built city thriving with life, cultural marvels and jewels of art. As a kid I was amazed to see these white statues standing above trees next to brown metallic ships landing in the streets, and droids lurking from the many green labyrinths.
But I must say, that other levels were good as well. TF Ship was a good action and true to the film, the swamps were more of a philosophical journey, as you make your way through the forest. I guess everyone remembers the small "valley", when you have to move a "box" and music from Episode V can be heard then.
I really enjoyed Coruscant level, and as Alexrd mentioned - the CorusMall. It was a good thing to imagine, taking into account, that there was not so much info on what the planet actually were besides the Senate and being the Centre of the Galaxy. The Roonan traders are always good to see, but so far they made their appearance in no other game.