The Thunder Shadow. I Always use this car, Its Super fast with its boost maxed out and you can take anyone down with it lol Trust Me, people can't touch or catch you in it.
Definatly the best car would have to be the Carson GT Nighthawk. It's top speed is about 175 with a boost speed of 205 (as stated by the Burnout Wiki). It is also extremely durrable so it will take a hit and keep going as well as being able to match the Olympus or any other heavy weight vehicle in aggressive power while being in a much faster and more nimble package. I have driven the Olymus Governor at full speed playing chicken with a Nighthawk who wasn't even using boost and It took me down without even getting a scratch on it. Also, what makes the GT Nighthawk special, is that (for the very skill drivers of it) it call pull off stunts better than the Toy 88 Special or even the Dust Storm. To sum it all up, The GT Nighthawk is a very awesome machine, far better than an other vehicle in the game. Though it may be a speed boost type, it can match even the best Stunt and Agressive types.