There must be some sort of prize for making the fastest car in the world. For a while the McLaren owned the title with the F1, then the Bugatti Veyron came along with its 1001 horsepower quad-turbo W16 engine with a top speed of around 400km/hr - never officially tested.
Bugatti Veyron
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And of course there is the current official title holder, the Koenigsegg CCR (392km/hr).

Koenigsegg CCR

A little bit after that Barbrus came out with the Barbrus TKR which supposedly goes up to 430KM/hr and does the 0-100 in less than 2 seconds (faster than an F1 car).

Barbrus TKR

While the Barbrus has never been officially tested, there is an American company now aiming for the title, with this:

SSC Ultimate Aero TT

The SSC Ultimate Aero TT is trying to beat the Koenigsegg CCR's official speed of 392km/hr. Winding Road reports that Junus Khan, SSC’s marketing director recently admited that the company has managed speeds of up to 389km/hr.

However the car apparently has only ever deployed 71 per cent of its available power to reach those speeds. The Americans look determined the beat the CCR's time, when will bugatti and barbrus authenticate their claims? In an episode of Top Gear, James May apparently took the car to over 400km/hr.