The authoritative body for aircraft records is normally Fédération Aéronautique Internationale and I believe the SR-71 is in Class C as a Aeroplane while the X-

The authoritative body for aircraft records is normally Fédération Aéronautique Internationale and I believe the SR-71 is in Class C as a Aeroplane while the X-15 is a Class P Aerospacecraft. So as far as "Official" speed records go the fastest "airplane" is a Sr-71. Its technically the fastest manned air breathing aircraft. There seem to be some semantic ambiguities between Aircraft, Airplane, Rocket, etc.