I've grown to hold affection for all the planets for KotOR I. I liked Taris for it's variety and because it was so big. To be entirely honest I was slightly disappointed that I didn't get to spend as long on the other planets. I remember first coming into Javyar's Cantina and quite liking the music, then wishing that I could walk up to the bar and have a drink while taking in the atmosphere.
Manaan gets an honourable mentioned for being intiguing. I liked the city feel of it, even if it was big and confusing I liked walking about everywhere. You also get to see the Republic and Sith when they're not allowed to fight each other... still at each others' throats, however.
I would have to say, however, that my favourite is Unknown World. I know it doesn't have the city feel that I like to it, it doesn't have the puzzles and quests that I like, but it's beautiful. As nice as the other planets are, Unknown World is the one planet where you can't help but stop and gawp at all the scenery, the water lapping at the shoreline you can stand in, the tissue-paper texture of the roof in the Rakata settlement, the crumbling pillars and stone blocks around the temple. I like to think that once I defeat Malak I come back and have a beach party.