I was thinking about this one today at work, and I can't decide between two, so as they tie for first with me, I'll post them both and let my readers sort it out (though I think I know which my current group will choose).
First, we have my very first fleshed out NPC... Silky Smooth. This pimp/hustler character was the first thing that popped into my mind when my Vampire players were looking for someone shady they could connect with. Silky might be part of the underworld, but he's anything but shady, with his flashy colored suits and "pimp cane" in hand. Though seemingly always on the pulse of every illicit trade in the city, Silky is ever known to have dealt in information. He will point you in the right direction, sometimes taking payment, sometimes offering his services for free on a whim.
To get the portrayal of Silky, I mainly drew from a guy I had come into acquaintance with when I was a teenager named Bobo. Most people who have spent any amount of time hanging out in Batesville, Arkansas have met Bobo. He'll freely admit to having spent time in prison for drugs and stabbing a dude, and is a self-confessed crackhead and vagrant. Take that core and sprinkle in some Katt Williams voice and Huggy Bear's wardrobe, and you just about get it...again, assuming you've met Bobo.
Anyway, he was always fun to portray and the PCs kept going back to him over and over again, even though they had other options, so I felt like they enjoyed the character, too.
The other NPC that leads the pack is Baron Althom. This character is mentioned as a benefactor in the 3.0 Dungeons & Dragons module The Forge of Fury, but had made more than that appearance in my attempts to run the Sunless Citadel adventure path and various other tries at Eberron campaigns.
Normally, I wouldn't put forth a character from a module as a favorite, but when first running the character, an eclectic art collector and Karrnathi noble, a good mix of the voices (and attitudes) that Peter MacNicol used for Dr. Janosz Poha, and Martin Short's Franck Eggelhoffer came to the fore and I ran with it. This character served in my long-running Eberron campaign as an ally and benefactor on more than one occasion, so I'm pretty sure he was liked by the party.
I've also used my "Baron Althom" voice and portrayal for various NPCs across settings, effectively making a multiversal character in my games.