What was the highest wind on the biggest tornado?

The fastest wind recorded in a tornado was 302 mph in the Moore, Oklahoma tornado of May 3, 1999. Other tornadoes in history may have had faster winds, but actualy wind measurements from tornadoes are rare.
On May 3, 1999, a strong F-5 tornado hit the Bridge Creek area SW of Oklahoma City, Oklahoma. Doppler radar wind speeds of 302 mph (486 kph) at a point about 100 feet of the ground. This was revised down from an earlier estimated of 318 mph (512 kph).
The highest wind speed ever recorded in a tornado was 302 mph +/- 20. This was in the F5 tornado that hit Moore, Oklahoma on May 3, 1999.

However, it is relatively rare for winds in a tornado to be measured directly so other tornadoes which did not have their winds measured may have had faster winds.