Bugatti 100P. In many ways the design was ahead of its time. It never flew so the theories weren't really put to practise. It for example used a flap mechanisim that is similar to the modern competition gliders. The top seepd claims however were wayyyy too optimistic, though as far as I can figure.
There's a group currently builiding a reproduction to fly that will be configured to the similar specs as the original one.