I don't know if I have a favorite. Aaron Allston write Strike Force, my favorite sourcebook of all time, and a host of Star Wars novels. Bill Slavicek is considered one of the foremost authorities on Star Wars and his SW RPG material was practically canon by LucasArts. Monte Cook cranked out a ton of stuff under the D&D masthead, and is now doing really interesting things with Numenera and The Strange.
But I'm going to go with Andrew Watt. I met Andrew in the first few months of being in college, and we became best friends from then on out. Andrew introduced me to the gaming community at our college, a group that became the social and emotional nexus of my life for the following years. Andrew was and is a kind, intelligent, creative human being who patiently tolerated my inevitably overly-flamboyant PC's: a Crazy in Rifts, a Smuggler in Star Wars, a Barbarian in Fantasy Hero. He himself would bring in meticulously-wrought PC's into whatever I was running, like the Celtic Demigod Bard who ended up in my very New Warriors-esque Champions campaign.
Andrew could crank out a ton of backstory to whatever he was working on, literally inches of pages of history, culture, even poetry to his worldbuilding. He would go onto contribute to the creation of Exalted and Scion for White Wolf, and wrote the introduction to Aria: Canticle of the Monomyth (which sounds so much like a title of something Andrew would be a part of).
He's got a blog out there now and you can see what he's up to these days, which includes apparently still cranking out a ton of material from his fertile imagination.