the fastest jet ever built is the NASA X-43, specifically the X-43A variant. The X-43A is an unmanned experimental aircraft designed for hypersonic flight research.
The X-43A achieved its record-setting speed on November 16, 2004, when it reached a top speed of approximately Mach 9.6, which is roughly 7,346 miles per hour (11,760 kilometers per hour). Mach 1 is the speed of sound, so Mach 9.6 is about 9.6 times the speed of sound.
It's important to note that the X-43A is an experimental aircraft and not a conventional operational jet used for commercial or military purposes. Other jets, such as military fighter jets like the SR-71 Blackbird, have also achieved extremely high speeds, but their top speeds are generally lower than that of the X-43A. Keep in mind that aircraft technology and capabilities continue to evolve, so there may be faster jets developed since I'm not interested in that topic as much as i used to and my answer might be a bit outdated.