What is the fastest plane in the world? The answer depends on the criteria. If you are discussing airplanes that can take off and land under their own power, the answer is the SR-71. If you are discussing planes that are carried by other planes to takeoff, the answer is the X-15. My criteria is the former.
The SR-71 was designed by Clarence “Kelly” Johnson. In aviation design, Kelly Johnson is a legend, an absolute legend. If the design world is a pyramid, Kelly Johnson is at the top.
The SR-71 is a derivative of the YF-12. The YF-12 was an interceptor prototype that carried missiles and was designed to shoot down invading Soviet aircraft. The US Air Force decided not to fully develop the YF-12. However, Kelly Johnson’s famed Lockheed Skunk Works modified the plane into a reconnaissance aircraft labeled the SR-71.
The YF-12 held the record for speed until July 1976. On July 28, pilot Eldon Joersz and navigator George Morgan set the speed record of 2,193 miles per hour (over Mach 3). The SR-71 likely flew faster than the record flight, but its top speed was classified. Joersz and Morgan celebrated their flight 40 years later: http://www.af.mil/News/Article-Display/Article/909026/sr-71-pilots-crew-relive-absolute-speed-record/
The rocket powered X-15 flew faster than the SR-71, but it was lifted into flight by a B-52. None the less, in 1967 Pete Knight piloted the X-15 to a speed of Mach 6.70. Knight was a fine American who later served in the California General Assembly.
For a list of the top ten fastest aircraft see: http://www.migflug.com/jetflights/the-10-fastest-aircraft-in-the-world.html
But for now the answer to the question, “What is the fastest plane in the world” is simply the SR-71. http://www.sr71.us/spd_run001.html