My vote goes to The Tyrant
Was that an eye-opener or what?
Here I am, min/maxed BH Sage, doing dailes on Belsavis when two lower levels are asking for help with a Heroic. With nothing better to do I decided to help them out, I skipped most of the missions as I leveled up so it would be fun to try something new.
We enter the dungeon and I pull out my Rakata healer, thinking this should be easy. I spot the champion in the middle of the area and decide to go for him. Shouldnt be a problem, I beat up lvl 50 champions on Ilum all the time and this guy is only a lvl 43. WRONG, I get my *** handed to me. What the heck just happened O_o? Granted that it is a champion but it's also a lvl 43 with only slightly higher HP than the council members on EV, NPC'es I beat up in no-time without taking any damage.
We try again and actually kill him this time but not until he has killed me and one of the lower levels first. Jesus, the guy hits like a truck!
So the Tyrant gets my vote as most BA NPC in the game, for the level he's intended for. 50's should solo him just for the challenge.