As far as scenery, I think Alderaan is the best. The thranta ride from the last spot in Glarus Valley to the Imp space port is probably the longest taxi ride in the game. But the views are amazing.
I've taken said ride even when my QT was available to just look at the mountains.
Voss is really beautiful too, and the caldera on Belsavis is its own brand of epic, especially on pvp servers.