American: It's hard for me to narrow down between the Big Three (Ford, Chrysler and GM). I like all three's muscle cars. But if I were to rank them, Dodge (Subsidiary of Chrysler) would probably be at the top, followed by Chevrolet (Subsidiary of GM), then Ford.
Mainland European: For super-cars, I think Koenigsegg is at the top of my list, followed by Bugatti. For luxury cars that are more for the middle class, I would have to say Audi, followed by BMW, then Porsche.
British: Aston Martin, followed by Rolls-Royce, then Jaguar.