When I read "biggest" at the head of this thread, I thought of power rather than weight or wingspan. Anything with a single piston engine to trump the R4360 powered Super-Corsair or DB2D for power?
For turboprops, I can think of nothing more powerful than those frightening tailsitters the Convair XFY and the Lockheed XFV. As for jets, I suppose the JSF is king of the hill.