If you are not talking about sports cars and cars that beat speed records etc nor antique cars that celebrities drove... Then the most expensive road car in the world should be bugatti veyron.
Rare antique ferrari roadcar they only made like 3-5 of these racecars and if they won a major race they can get around 10+ million or so
Not too expensive at all for the world's costliest car. Guess I'll have to have one custom-built for me when I become a billionaire