Dromund Kaas is by far my favorite planet. I love the jungle setting and arriving at Kaas City for the first time, seeing this massive city, not unlike Coruscant, but in my opinion a far interesting place, is just amazing. There's also quite some interesting quests to do here, including one with ties to Revan.
I also picked Corellia. The developers outdid themselves building Coronet City. And the tubes, the tubes!