Here's my two credits: (1-3 scale)
Pereagus: 1. Very boring. The only enemies up until the very end are mining droids.
Telos: 1.5. Alright I guess. Not the most exciting planet but it wasn't bad either. And plus, that's where I get the Mutakai Apprentice robes so it's all good.
Dxun: 3.0. There's too much stuff to do here for it to be boring. Especially part two.
Onderon: 3.0. My favorite. Definetly the best IMO. I didn't like the freaky looking twi'lek holograms in the Cantina, but other than that it was all good.
Nar Shaddaa: 2. Not my favorite place but it's a good [moon]. It's boring up until the whole bounty hunter thing starts up.
Dantooine: 2.5. Dantooine.....Fighting countless Kinrath get's old but all the crystals and stuff you get from there is very handy. Exploring the ruins of the destroyed academy is pretty tight.
Korriban: 2.8. I'll always like Korriban but the lack of things to do here keeps it from getting a 3.0. In kotor 1 it was without a doubt the best but, as someone mentioned, it looked like it was rushed a bit. However, the actual Academy seemed more 'fun' than Kotor 1. Too bad you couldn't purchase ancient Sith history books from the library. And I really liked Ludo Kreesh's tomb. That was the best part of the game IMO.