My favorite planet is Belsavis, my IA storyline was extremely well done here. The planet felt explorable and the balance between interactive and environmental elements vs. combat and mission objectives was better than other planets, IMHO.
The Belsavis daily missions that return me to the planet are good and I would like to see more of them! I like the Imperial cantina on Belsavis, even though it is small, it feels realistic and cozy to me.
What is your favorite planet in The Old Republic and why?