Difficult choice, as they are from different era's of aviation development and both at the cutting edge from their respective designers.
The Spitfire, Mitchell BTW thought it was a terrible name, didn't have a straight edge anywhere on it, was difficult to build and really started performing when Stanley Hooker at Rolls Royce got to work on the Merlin turbocharger. Beautiful looks and sounds and I've yet to meet anyone who flew it that didn't enjoy the aircraft. So could be a candidate.
The Concorde, the only viable SST commercial aircraft to be built and operated, equal to getting a man on the moon, and finished off by the French when they refused to release the aircraft's design authority. Used occasionally by the military for high level supersonic intercepts. I can remember the first commercial flight from LHR, not a car or person moved, the whole area was at a standstill for the takeoff. Can't imagine another aircraft having that affect on us lot? Beautiful, beautiful looks, sadly missed and maybe never renewed.
My vote would go with the Spitfire, probably the Mk IIa, still very clean looking, retaining much of Mitchell's original lines and before it was given the steroids in later marks.