I would go for KOTOR1 easily. The story and characters feel more "Star Wars" like.
I prefer the setting and planets in KOTOR1 as well. They are much more visually beautiful and this a huge factor for me.
KOTOR2 has too many ridiculous concepts to my taste (ridiculously powerful villain, immortal villain, etc.).
I would however have much more trouble to pick a favourite between KOTOR1 and original SWTOR story-related content.