I'd say they Lamborgini Gallardo. The reason why i reckon its the best car is that its an all-rounder, when fully upgraded, it gives the car a high amount of speed, acceleration and handling. When versing an opponent, u'll start off with a great acceleration, if ur opponent is ahead of u, u can sometimes catch up with ease and when u approach a tight turn, it'll also be handy as u'll glide through it easily.
I said that in the PC quiz :P