Help us out a little:
Define best?
Do you want something to annihilate the competition just for game completion?
Do you want close tight racing for as many A-spec points as possible?
Have you already tried something, and why did it not work?
There's plenty of options here, sometimes a car that's not competetive against one lineup of AI opponents will wipe-the-floor with another set!
Easy wins can be had in: AC Cobra or Shelby Cobra once you learn to control those cars.
Competetive wins: Alfa Romeos, Isuzu 117 Coupe if you search for a lineup that doesn't contain the Buick Special, AC Cobra, Jaguar E-type, Chevrolet Corvette or Mercedes 300SL.
Avoid: Fiat 500, Citroen 2CV if you're looking for good points as these will drive the points total lower.