To me, defining the best looking car means a style that appeals to ones everyday desire, not necessarily the most exotic or rare vehicle. While I think that Zondas, McLarens and Enzos are very attractive cars, I wouldn't call them the best looking cars ever. If popularity and style longevity are any sign of a good design, I'd say the award for best looking car would have to go to the Porsche 911 line... I can't think of any other vehicle that has persisted through the last 50 years with the same virtual style and been so successful. Other cars that have been in production for (nearly) as long have gone through several distinct body restylings (mustang, corvette) that leave the evolution to modern styles somewhat choppy, whereas with the 911 line, it's fairly subtle changes with each iteration.
Just my $0.02 though.
Just my $0.02 though.