Favourites: I'd have to say Voss (actual challenging fights) and Nar Shaddaa (city-planet).
Least favourites: Taris (quests go on for bloody ever, on my 2nd toon I'm currently playing I plan to GTFO as soon as I'm done the class quest series there), and Belsavis (Belsavis has bloody annoying navigation system, too many walls/rocks/cliffs and forcing you to navigate a freaking maze for half an hour to get to something that's actually only 1km away)