Well I suppose by the end of the war it has to be the Mustang, but remember the Mustang wasn't really any good at altitude until Rolls Royce got their hands on it.
Also remember that during war the inventiveness of scientists and technicians expands hugely. Nothing like the possibility of catastrophic defeat to concentrate the mind.
World War Two started with pistion engines and quite a few bi-planes and ended up with ballistic missiles and jets.
So comparing the Mustang and Spitfire (even the later Marks of the Spit) you're ignoring the fact that the Spit was around earlier and was good enough to help win the BoB. Had it not done that then for Blighty at least, the appearance of the Mustang wouldn't have mattered that much. (And, as mentioned, it was the Brits that made the Mustang what it was.)
All the above subject to ropey memory.
Edited to add - Bugger. The question was about looks!!!
So - Easily the Spit.
