My father-in-law flew the RP-63. This was an armor plated P-63. His job was to mock attack B-17s (Axis style) so that gunners in training could get some live fire experience. The gunners would shoot 30 cal. machine guns with frangible ammunition at his plane. When they scored hits, a light in the spinner would light up and a hit counter on the instrument panel would record the hit so that he could give feed-back over the radio to the gunner. Bill said that he always knew when the instructor was demonstrating because he would get nailed right away.
Was the RP-63 the "Toughest US fighter of World War II"? Probably not, but I can't imagine any other plane took as many hits without needing repair.
If we limit the question to fighters only, excluding attack aircraft, I think the Tubby-Cat, Hog and Jug would come in a tie that could be debated, but never decided. Include attack aircraft and the IL-2 stands alone.
eskimo