The most beautiful car ever, to me, is the 1986 Ford Taurus, because it changed automotive design like no previous vehicle, with the possible exception of Ford's Model T. It was the rare case, the only case I can think of, where evolution made a leap rather than a slow transition. It was also beautiful because it virtually saved Ford Motor Company in the 1980s and it showed that America could compete after twenty years of trying. To me, the 1986 Ford Taurus is beautiful because it was perfect.
The best-looking car ever built is the Ferrari 456.