Although I believe this is too broad a category considering the extraordinary accomplishments of so many, I would have to say Clarence Kelly Johnson, whose work pushed the aircraft performance edge and envelope to an almost unimaginable degree.  This is most certainly demonstrated by the Lockheed P-38 Lightning, the U-2 and the mind bending SR-71 Blackbird.   Very few have so fired the imaginations of so many both in and outside of the aerospace industry.  Following would be Jack Northrop (P-61 Black Widow, XB-35, XB-49 and by extension, B-2) Kurt Tank(FW-190, TA-152), Dutch Kindelberger (P-51), Sydney Camm (Hurricane, Typhoon and Tempest) and Paul I. "Pappy" Gunn( Upgunning of A-20, B-25 and development of "Skip bombing" technique) Frank Whittle (a category of his own: Jet engine creation and development) and last but not least, Prof. Barnes Wallace (Geodectic aircraft structure for Vickers-Armstrong Wellington; Avro Lancaster "Dam Buster" variant with "Upkeep" cylindrical spinning bomb.

As I said, this is a very, very tough call as to who constitutes the "Greatest".  Perhaps a composite representative entity? MDS, GHH, LLC  www.emotionreports.com