Really depends on how you define fastest.
Laptime speed?
Topspeed?
Raod legal car or not?
It sounds to me that you are looking for rhe car with the highest topspeed.
As far as i know this is the 'Thrust SSC'.
Which by no means is a road legal car. It reached a speed of 1.227,985 km/h.
The fastest road legal car would be the 'Königsegg Agera RS', reaching 457,53 km/h (284,3 mph).