My personal favorite is Voss. I love the scenery there: the constant autumn colors and trees. I also love the general story arc there. I love the fact that it's a completely different culture and exploring the gray force culture of the Mystics. (I'm also reading The Dawn of the Jedi comic books right now that explore the original Je'daii, which I find very fascinating and add to why I like Voss more.) My Consular's storyline was a little bit of a sidetrack there, but the archetype of the Consular fit well with the whole planet. My Inquisitor's storyline was awesome there!
I loved finding out more about the Jedi on Tython. Korriban and Dromund Kaas were wonderful for finding out more about the Sith, especially the tombs and temples. Alderaan was visually stunning; I just wish it explored the house politics/history a little more. Ilum was really cool because of the unknown-to-me factor and how it fit in with the Republic and Sith. And one of my other favorite planets (a lot of you will think I'm nuts)...Tatooine. I thought it was a beautifully done desert planet that captured what was shown in the movies very well; the story arc wasn't bad either. And hey, it has a friggin' balloon ride!