As of 2010, the fastest plane in active service was the MiG-25 Foxbat, developed for the Russian military with a stated top speed of Mach 2.83. However, several historic aircraft have gone faster, and a number of experimental aircraft have also broken this speed record. It is also important to note that the top speed of many military aircraft is classified, and it is highly likely that the identity of the fastest plane is not known and will never be known unless the military opts to declassify it.
The military has an interest in keeping such information classified. No nation's military wants to alert other countries about the full potential of its capabilities. While many experimental aircraft are tested publicly and profiled to alert other countries to the fact that certain types of aircraft are in development, their top speeds and full capabilities are not released because this could threaten operational security in the event that the aircraft entered service and was used in a military conflict.
When exploring aircraft speed records, one must consider what qualifies as a “plane.” The space shuttle, for example, exceeds the speed of all aircraft, but it is technically a spacecraft, not a plane. Likewise, spaceplanes which are capable of high speeds are also classified as spacecraft, and thus can't be considered in a record. If rocket engines are acceptable and a plane doesn't need a pilot, the fastest plane is the X-43, an experimental aircraft developed by the National Aeronautics and Space Administration (NASA), which set a record of Mach 9.6 in 2004.
The fastest piloted plane, again if rocket engines are allowable, is the X-15, which reaches a declared top speed of Mach 6.72. The X-15 was retired from service in 1970 as additional planes in the X series were developed. If rocket engines would disqualify an entry, the SR-71 Blackbird, also a retired aircraft, is the fastest plane, with a publicly recorded top speed of over Mach three.
In terms of passenger aircraft, the fastest plane ever developed was the Tupolev Tu-144, which reached a top speed of Mach 2.35. However, this aircraft proved to be unsafe in service and it was quickly retired, allowing the Concorde, with a top speed of Mach 2.23, to hold the record for the fastest passenger plane. It should be noted that the Concorde rarely reached this speed in active service due to limitations on the aircraft and that speed records in general are recorded under ideal conditions and many planes with stated top speeds rarely actually attained those speeds.
