A Bizzarinni 5300 because of it's sexy lines, big HP and good handling.
That being said, any car of a similar type would be a real treat as I have never driven anything of the like.
Other than my '86 Jaguar xks-c, but not enough hp in it....yet....
That would include any mid engined exotic, like a Ferrari 250 LM, or anything like it.
I have also never driven anything older than my '56 Plymouth. I'd love to drive a '30s Packard or Cadillac.
Also I think it would be fun to drive a cab-over tractor/trailer without the trailer.
Vintage, of course.