Agreed on Tython. I also really like Tat, Rishi, and Dromound Kaas. The developers did a great job at capturing the atmosphere of those 4 planets to me.
I used to hate DK, but I've came to love it more probably because I've been forced to spend so much time there
As for a planet not in the game, I love Naboo. It's the most iconic Star Wars planet to me and I'd love to see it in the game.