HK-47, Bastila, Canderous, and Jolee. These were the best NPCs and Canderous by far the best tank.
But I have to say that I never felt overly attached to anyone. I didn't care whether they lived or died.
Jolee was the most interesting character from my point of view because he was neutral and didn't push you in any direction - he left the decisions up to you - light or dark. I liked him, but I also had no problem killing him in the temple, either.