I'm a bit surprised to be honest--Taris? Though to be fair I've played the game so many times that Taris has reached the point of "chore" for me. The only worse setting in the two games is Telos IMO.
I'll go with Korriban. Great aesthetics and atmosphere coupled with providing more lore/information on the Sith and the Sith-Jedi rivalry than all the rest of the game combined, not to mention all six movies and probably quite a bit of EU literature as well.
Tatooine and Manaan both rate highly for me as well. And the Rakata homeworld has grown on me since I started paying more attention to it rather than just racing to get to the Star Forge.