Definately Tatooine + Taris. For Taris, although it gets boring quite fast for the veteraned player, is designed very well and displays the Dystopian society in the world of Star Wars. Then you get to see it torn apart by the sith like the bung-hole it is. Oh joy.
Tatooine also made it, because it's Tatooine. Being able to Swoop Race in the sands and slaughter Tuskens makes the time there enjoyable. And the Sandcrawlers to die for!