KotOR 1, hands down. KotOR 2 just seemed . . . empty to me. The darker plot is intriguing, I'll admit, except that I don't play Star Wars for darker plots. But all that aside, I never felt any sense of real resolution, even with TSLRCM. That's one big reason I was excited for Malachor VI (I hope it gets finished one day, it looked so great). Another big thing that bugged me about TSL was the voice acting. Atton was a very well written character, but Nicky Katt is nowhere near the actor that Raphael Sbarge is. (my name would've actually been CarthsGirl if I had realized when I signed up that this wasn't just a TSL forum . . . then again, it's a dumb name either way . . .)
But basically, TSL felt weird and unfinished. I never did quite figure out how Kreia planned to destroy the Force or where my character made her big choice. And Malachor was a ghost town with no real explanations of what exactly happened to some of your crew you've been investing in the whole game. Comparing unmodded games, I would never recommend TSL. Comparing the unmodded KotOR 1 with TSLRCM, I still say KotOR 1 is better, but TSL can be enjoyable.