The Harbinger, Begeren Colony, and Jung Ma for me. I'm on so many mainly thanks to a few random alts I made to play with friends who no longer play.
As for mains, I have a lvl 42 Sith Sorcerer and lvl 36 Powertech on Jung Ma and a lvl 40 Sith Juggernaut on Begeren Colony. I also have a lvl 20-something (I'd have to check) Jedi Sentinel and lvl 20 Gunslinger on Harginger. And then some lowbies of all the other base classes here and there. I play the game exclusively for the stories, in-game role playing, and companions so I've never found much of a drive to rush to level cap. Though this new expansion has given me enough of a push to start playing again. I could just take the lvl 60 boost, but that skips all of the character building and story leading up to it. I'd rather play my way there.