Well, there are different aspects to be taken into consideration.
There's favourite and there's
favourite. Know what I'm saying? Probably not...
Allow me to explain.
I dig Keldorn. I think he's the best NPC out there. I mean, give him Gauntlets of DEX and Carsomyr, and there isn't much that can stop him. Add that his kit gives him immunity to charm and hold, and he's the perfect flayer-slayer. Plus, his personal armor grants Free Action, plus, he can Dispel Magic and True Sight, which are 2 invaluable spells.
On the other side, I adore Jan for his personality. His interaction with the other NPCs is, simply put, hilarious ("Hey Haerry, I have an idea for a play!" lol Angus the Giant Beaver & Gungan the Hormonal Moose

, and "You'll go far, you know, even further maybe, than Uncle Bifflechips, though he was thrown by a catapult, so I suppose I shouldn't count him..." ROFLMAO

). And Boo likes him... so he says...
So there you go, my 2 favourite NPCs. Keldorn for the effectiveness, and Jan for the character.
Now if someone would only care for my opinion. Hmmm, really should think about that before I post something.
Oh you! He asked for it!
Hmm, then I suppose it's alright...
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GameBanshee, where I shall be a moderator, one day...