Taris is my favorite, because I'm a city person and it's the first planet you go to, so when I was there I was still overawed by how cool the game was. I still loved the game when I got to the Rakatan world, but I'd gotten more used to the awesomeness.
I also liked Manaan, because I actually liked the Selkath. Don't see what everyone has against them.
And third place for me would be Korriban, because you can infiltrate the Sith Academy even as a LS character. I mean, how cool is that?