KOTOR gets my vote.
Sith Lords drags on far too long before you get to choose where to go and finally get a lightsaber, and the two planets you have to slog through beforehand aren't nearly as interesting as Taris. Please devs, in the future ditch the idea of having to "earn" a lightsaber. The worst part of every SW game where you play as a Jedi is the part before you get a lightsaber, and SL is no different. Start the game in high gear and don't look back.
And with the exception of Nar Shaddaa the planets in SL aren't as interesting, atmospheric, or cool-looking as the ones in KOTOR, and even in the vertical city there was far too much running around in bland, indoor/greyish areas. Two of KOTOR's planets were reused but there's even less to see and do in them this time around.
Couple that with the general lack of improvements to things like customizing appearances, load times and frequency, and a bunch of other things small and not-so-small, and you're left with a game that falls short of the original in more ways than it outdoes it.