Alderaan.
A simple reason, but I found it quite nice. It felt all majestic with all the snow and mountains, and the story and missions for Consular class made it seem like it was where I should be..? if that makes sense.
Late one night I went and explored the entire map and found some really interesting places..found the Sith Base way up north and felt like I was trespassing...
Still my favorite but I still havent seen Voss, or Ilum ..or Belsavis..