Greatest athlete overall ? I'm fairly sure cycling is far more punishing than any of the other sports mentioned (including boxing), so I guess I'd have to go for Merckx.
Greatest American sportsman ? I would have to go for Ruth - he so clearly outclasses anyone else who has ever played baseball (in a way Jordan doesn't in basketball). Only Don Bradman compares in terms of being so clearly head and shoulders above all the other (of any era) proponents of a sport.
So Merckx, Ruth or Bradman.
PS Why is Walter Payton on the list ? Is CaptainKeyes originally from Chicago ? An excellent running back, but it's very hard to be confident argue that he's the best RB of all time (Brown ? OJ ? Sanders with his 5.0 average ? Emmitt Smith, perhaps ?)