Can only speak for K2, but my preferences were Atton and Disciple, with occasional doses of Bao-Dur. I don't think I have any logical reasons for my choices - I just liked them. Atton did well at either ranged or Jedi, and Disciple could make a good tank Jedi.
I really wanted to use T3 more, because I also liked him (cuteness factor), but whenever I tried, he just felt useless. Sure he could help out via ranged combat, but not enough.
HK was ok, but beyond his funny sarcasm, I didn't think he was that great in combat.
Visa's was fine as well, but I didn't find her entertaining - she was a sort of stick in the mud.
Hanhar really got on my nerves with all that growling and his backstory-whining. I don't like Kotor wookies at all.