This is obviously a subjective question, and off the top of my head I couldn't think of a list that would do justice to the amount of beautiful cars out there. I do, however, have a weakness for curvy, classically-proportioned sports cars, and in that vein I'd definitely put these near the top of any such list:
Ferrari 250 GTO
Mercedes-Benz 300SL Gullwing
as well as any number of pre-War coachbuilt cars, namely the works of Figoni et Falaschi. Damn.
For me, bulging torpedo fenders; sleek, low-slung greenhouses, and lots of curves are often the perfect recipe for a stunning automobile. Hence the reason I tend to overuse them in my own drawings.
Obviously, this is not my definitive list, because for me that would be impossible.