The slowest to fastest ratio plane
The radio-controlled model sailplane that recently exceeded 540 mph while dynamic soaring has to be a contender.  Minimum speed in this flight (immediately after launch) may have been as low as 10 mph, which probably could have been sustained by staying on the front side of the ridge.
Link to You Tube video-- 
                
            
Though any plane capable of routine flight at 0 mph would automatically win I suppose-- e.g. pretty much any hang glider or sailplane given the right conditions.
Or did you mean airspeed?  The radio-controlled sailplane in the video may well be one of the top contenders.