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Benny Löfgren:
In my mind, there can only be one answer to this question: The North American XB-70 Valkyrie, the planned American triple-supersonic strategic bomber. Its unique downward-foldable wing tips explored a principle called compression lift to achieve its high lift-to-drag ratio at high supersonic speeds. Not only did it have bad-ass performance, it looked absolutely stunning.
Two examples were built, one of which tragically crashed in June 1966. There are a lot of things I could write about this amazing aircraft, but there are already several good answers here on Quora already detailing its history — the Wikipedia article about the Valkyrie is a good start for further reading.
Marcus Chalumeaux-Marly:
The AMD-BA (now Dassault Aviation) Super Mirage 4000, essentially two Mirage 2000 glued together. Double the MTOW (32 tons), double the thrust 19 tons with two M53 engines), more fuel and payload capacity. To give you an idea of the size of the aircraft, you could fit a Mirage 2000’s terrain following nose radar (the Antilope 5) into a fuselage pod that looked tiny compared to the Mirage 4000. Obviously, it was a performance beast and was a serious challenger for its main competitor at the time, the F-15. Both were interesting Saudi military planes, but due to political maneuvers, the Super Mirage never made it through. As for the national market, the French Air Force didn’t want it for budget reasons. However, the Super Mirage’s legacy lives through the Rafale, which is a fantastic aircraft in every respect.
Source: thepointsguy.com