K1: Taris. K2: Nar Shaddaa. And no, it's not because I like the city (which I do
I like Taris because...well...really hard to explain. It has the perfect combination of main quest and side quest, and both are equally entertaining.
A close second would have to be...does the Leviathan
count? I didn't really like most of K1's planets. The main quests were severely lacking, in my opinion. Kashyyyk was pretty good, and Manaan was all right in that area (but not in many others). But I absolutely despise Tatooine, Dantooine, and Korriban.
As for Nar Shaddaa, I love it for the ending. Starting with the message from Visquis, all the way up to the escape from Goto's Yacht, it's very cinematic. The thing really feels like it belongs in a Star Wars movie.
And a very close second for K2 is Peragus. Yes, you heard me right. I just loved the whole mystery behind it the first time, and for me it never gets old.
(And yes, I know that the poll was only for K1, but I felt like talking about K2 anyway.