My favourite is definitely Manaan, mostly because of its beauty... seriously I totally calm down when I turn back to Manaan just for fun.
and I also loved that most of the time is spent with "diplomatic" quests and less with fighting. Selkath are quite interesting creatures, as much I used to hate their speech, as much I love it now. The only thing I hated were the firaxa sharks >.<
My second choice is Taris. I think this planet has the most interesting quests so far, especially the time spent in the Undercity, the whole story, the rakghouls, etc. Uhm and not to mention the music in Javyar's Cantina - absolutely brilliant