I have a similar deal except I have a few more :P Serana, Jarl Balgruuf, Farkas, and Vilkas
Serana as you said Hunter is just a pretty unique character and is one of the more beautiful characters in the game (design wise). Most of the NPCs are the same old robot with a slightly different look. Serana feels different when you're around and the dialogue with here does sound real.
Jarl Balgruuf....first Jarl you meet and he's just cool the way he handles things.
Farkas/Vilkas: Sad story + Werewolves + Epic skills = Win
Some obvious ones:
Some less obvious ones:
But if you made me pick one, it'd be M'aiq the Liar. Wise Old Khajiits for the win!
M'aiq the liar is more of an easter egg, he has always been there to make funny comments that answer / address / plain make fun off some odd requests or some obscure or even missing features in the game.
He was never meant as a serious character
Myself... well, K'zargo deserves an honory mention... he's the only npc who cares :'(