It is probably not the single most greatest innovation (not with jet engine and instrument flight, etc.), but it greatly effected performance during the prop era.
The entire field of electonics applied to the airframe industry - from R-Nav to GPS to egt/cht/epr sensors, to thermoclouples, to CAD to computer models such as NASTRAN to VSAero and flight simulators.
Make mistakes that are caught by the models, improve on the design, learn to fly w/o burning fuel, control an aircraft without a pilots, etc, etc.
Nobody has mentioned the helicopter yet. It can hover, take off/land in an area the size of a small parking lot, and is more practical than the airplane VTOLS. These uses have been incorporated into almost every role in civil and military aviation.