I didn't even have a second thought when trying to decide who my favorite NPC of all time is. His name is Gregor.
Gregor is a Moradin-loving human fighter. He's stronger than an ogre and has a good sense of humor. He can throw a hammer like no other (I really think he could give Thor a run for his money). Thanks to the robe of eyes he wears, he's rarely ambushed. Despite being an npc, he started at 1st level like the party and wore the same (non-magical) chain shirt for at least 9 levels. His armor actually has a bit of iconic status in our hearts. He's also my character Ulic's best friend. The pair met while Gregor was working for a thieve's guild. He was the protector of a rogue named Quentin, who was played by the DM's uncle, Daniel. He and Ulic became friends and when Daniel stopped playing Gregor started adventuring with Ulic.
I've never really thought of Gregor as an npc. To me he's always been Wirty's character (which makes sense because Wirty was the DM). I still remember driving to town with Wirty when we came up with the idea of D&D: The Musical based off of our characters. I can't remember all of the lyrics (and I really wish we'd recorded it), but I remember Gregor kept interrupting Ulic and singing "Hail Moradin! Hail Moradin."