Fully modded Saleen S7. I've used for Saleen S7 Club (obviously), Supercar Festival, Premium Sports Car Lounge, and EL Capitan 200 Miles, and aced the competition in all 4.
Go for the Ford GT. It's one of the cheapest supercars and because I didn't read the rules properly, I ran it bog stock except for oil. It still creamed the lot of them.
It may be dodgy on the brakes, but braking earlier seems to solve the problem...