HK by a long shot.
The DS storyline is worth alienating all your goody-two-shoes party members just to hang with HK in TSL. Best dialogue in either game, period. Sure, he's kinda useless without a super beefed-up repeating rifle with all the trimmings, but then TSL is so easy your PC should really be able to slaughter pretty much anything on its own anyway. The other party members in KOTOR 1 & 2 all have their own quirky charm, but (if you've paid attention to my posts) I hate Carth by far the most. His constant whining about trust issues got so aggravating I just sent him to the ship for the rest of the game as soon as I got Mission and Big Z. Big disappointment that I had to have him with me on the Leviathan...
Be considerate to others or I will bite your torso and give you a disease!