That is difficult to determine exactly. The strongest winds ever recorded in a tornado were 302 mph in a tornado that hit the Oklahoma City area on May 3, 1999. However, very few tornadoes get their winds measured, so others were probably more violent. There are a number of candidates for "most violent tornado", such as the Tri-State tornado of March 18, 1925.
However, the tornado that seems to stand out is the tornado that struck the small town of Jarrell, Texas on May 27, 1997. This F5 tornado moved over the Double Creek Estates subdivision at peak strength and essentially erased it.
Unlike other tornadoes this storm did not leave a filed of debris and mangled trees: virtually everything was swept away and pulverized. Houses were wiped clean off their foundations, trees and power pulls were sheared off at ground level, grass, roads, driveways, and over a foot of topsoil were stripped away. The debris found from this area was finely granulated.