I used the Vette Z06 for the American races, easy wins all around. I also grabbed gold on a couple of the all manufacturer races with it; I haven't seen a better car for Daytona yet (the Vette will pass 200 in the straights, while the F430 maxes at 194 and the GTR takes too long to get up to speed).
The Scooby has got to be a Subaru Impreza... And the new shape is an ugly, ugly beast.
I used the NSX to capture gold in one of the Japanese races; it was a tricky run weaving through the pack, but when you get open, you can really drive the hell out of it.