I have to agree with Sean Patrick: the fastest fighter jet is or was the YF-12. The question itself would left room for fantasy jets as well, but I will stick to a plane that was really built and that really flew. The YF-12 was not only the fastest, but also the biggest fighter jet ever built. Some sources also said that it would have been the best fighter jet in its time- flying with a speed of 3660km/h in an altitude of more than 27000 meters would have make it nearly impossible to shoot down- this was even about 5000 meters higher than a Mig-25 can fly.