The Formula Gran Turismo is the best car in Gran Turismo 4. It is the only Formula 1 car in the game, and it can be won by winning the 24 hour of Nurimberg* (spelling is probably wrong)(yes it is) endurance race. *Nurburgring nordshlief
If you have unlocked the dream car series, have an extremely fast car like a tuned Enzo Ferrari, Mclaren F1 or the race-car you get from the Audi dealership, you can get around $450,000 per championship. If not, then I have know idea.