For those of us in the UK: Don Turnbull
Not only did he have an impact on D&D in general (the Fiend Factory column in early White Dwarf which leant its weight to the original Fiend Folio, as well as the excellent Sinister Secret of Salt Marsh, Danger at Dunwater and The Final Enemy, plus Beyond the Crystal Cave), but he had a huge impact here in the UK, where he was head of TSR UK and editor of Imagine Magazine.
He sadly passed away in 2003, but is not forgotten.