Favorite NPC is officially
Serana.
Spoilers below: She's one of, if not, the most developed NPC character in the game currently. Aside from discovering she relates profoundly to my own female vampiric character, I was pleased to see how Bethesda actually made her a true companion compared to the others. She's thrice as developed as Lydia, Aela the huntress and even the Blades' woman(name?). In every quest, she makes a comment on something that ties in with the enviormnent. And the story is amazing. A vampiric woman with parental issues. Never thought it would be this awesome. Everyone in the forums and who played Dawnguard seem to like her.
Though I'm not buying how sweet and "nice" she is, though I love her character. She is a worshipper of Molag Bal to the point of going through a sexual and degrading ritual to be transformed into a pure-blooded vampire, and still considers it a gift upon being asked for a cure.
She is also somewhere around a thousand years old or more, from the 2nd Era, as for a period then there was no Empire(which she is shocked to discover after all this time that Cyrodiil was the seat of an Empire after being locked away). She never uses her Vampire Lord form no matter how cruciel the circustances are, and showed that there is more to her power/abilities when she is completely unphased by the Snow Elf's damaging attack that wounded himself and flung the player to the floor senseless. She never displays to the player a fraction of her true capablities, just standing back and throwing spells, until she looses her temper and gets angry the with the Snow Elf and picks him up by his collar to choke him to make threats.
She is also really eager to turn the player into a Vampire, or see him/her turned into one. I came to see that she also gradually grows to like the player, and how protective she gets, to the point of making out to her father that protecting the player is a big part of her betrayal. And I noticed that Harkon made a remark about her keeping the player as a pet, as though it wasn't a big suprise or new, and how she didn't really argue.
Frankly, since vampires are infamously manipulative I wouldn't be surprised to see if she isn't just a hell of a chessmaster.
An alternative way to look at it is that she is pretty much the player's lover if you stay together to adventure. There should of been a marriage option, there really are vibes to the back-and-forths between her and the player, and her comments as well. I wonder if Bethesda didn't include the option more because she is a vampire, and an ancient one at that, who may not want any part of a marriage before Mara. In any case, a Molag Bal marriage ceremony would be appropiate(sans the degrading ritual).