I would have to say that a very critical milestone comes from a man whose name is not a household name, nor has he ever really had the popularity or notoriety that was bestowed upon the Wright brothers, yet his contribution to aviation has been unsurpassed by any other (What are the 20 most critical milestones in aviation? Feb. 17 issue).
The man I am referring to is Glenn H. Curtiss and his contribution to the world of aviation is the aileron. Nearly every single airplane that has ever flown has some form of a hinged control surface that is used to change direction and heading. I believe the man himself is a pillar of early aviation ventures, innovative in combustion powerplants to propel motorcycles, dirigibles and experimental aircraft, improving aircraft controllability, and promoting aviation in general.