To be honest, it's hard to pick just one. They're all interesting cars.
Gt citroen race car : it's such an odd car, in-car view is bizarre but neat-o at the same time.
Red bull x1 : This one is fun for testing your reflexes.
Enzo: I'd have to say that the Enzo is a misunderstood car. Many people have the thought process of "build the car to max power & go". Once you've got this one tuned right it's a track monster.
458: all-around fun car, whether drifting, dragging, or grip racing. This car is a great performer!
Just my two cents.