Ravenloft AND DragonLance? I think you could give it to him (and Laura Hickman for the modules and Margaret Weis for the books) as the biggest influence after the creators of D&D itself, chronologically. Hickman influenced two tracks of early to mid-'80s D&D - adventure design and best-selling books that ushered in dozens of other books.
Then I think you have to go to Ed Greenwood because of Forgotten Realms and the D&D focus in the late '80s and '90s - settings.
Then I think you have to go to Ed Greenwood because of Forgotten Realms and the D&D focus in the late '80s and '90s - settings.