I have the same answer, so I'll give an honorable mention to Kenneth Arrow.

He used more modern mathematical notation to demonstrate several extremely important results: i.e. collective decision making cannot be done fairly and the market clearing price will always be at an optimally efficient level.

He doesn't get enough credit, imo, because his indisputable conclusions are very inconvenient for the wannabe technocrats who dominate the profession and because his mathematical approach is not appreciated by the Austrians.