Dave Arneson
Rob Kuntz
Tim Kask
Mike Carr
Lawrence Schick
David C. Sutherland III
David Trampier
Tom Wham
Erol Otus
Tom Moldvay
Frank Mentzer
Steve Marsh
Bob Bledsaw
Paul Jaquays
Mike Mornard
John T. Sapienza
Steve Perrin
Will Niebling
Dave Arneson
Paul Jaquays
UK Crew: Turnbull, Bambra, Gallagher, Davis (almost all they produced was amazingly good)
Ed Greenwood
Tracy Hickman
Monte Cook
Erik Mona
Wolfgang Bauer
Bruce Cordell
Chris Pramas
Marcus Walker. If he hadn't run those great games early on in my gaming career, I would never have stuck with D&D as a hobby over these last two decades. Thanks, Marcus.
Oh, and good DMs like Marcus everywhere. Without them, the game wouldn't be great!
Bruce Cordell, Monte Cook, James Jacob, Leonard Lakofka, Trampier, Erol Otus, Wayne Reynolds... the list actually goes on for quite a ways, both for designers and artists.
Dave Arneson is in a different league from the rest because he had a hand in the foundation. The combination of the basic rules - classes, levels, hit points and armor class - the default adventure - the dungeon bash - and the monsters and magic items make D&D what it is.
Many others have done good work but they didn't make D&D great. It would still have been great if they had done nothing.
Charles Ryan, for actually participating on a daily basis in the forum dedicated to his work (d20 modern is close enough to D&D as far as I am concerned).