For me it's got to be Imperial Taris. Just the whole Thana Vesh thing kept me hooked - Republic Taris was a bit of a chore, one of those feelings you get when a planet just.won't.end. Imperial Taris on the other hand was a delight.
Republic side I'd have to say it's a toss up between Tython and Hoth. Tython was pretty trpical but welcome nonetheless, but I really liked the part on Hoth where you