Any one who steps in the ring with Marciano is taking a dare and showing some courage, but for Louis to do it at his age and physical health is showing courage like never before.
Max Scmeling is one of the most underrated champions ever. Bert Sugar, ESPN Boxing historian, says that he is the most underrated champion ever. He owned one of the best right hands in history, and sure did give Louis a hell of a time with it. But what did Louis do in their second fight? DESTROYED HIM.
Jersey Joe Walcott was one of the greatest heavyweight champions of all-time. There's no doubt about that. He gave Louis problems, but Louis won the first fight, and gave one of his best performances en route to decisioning him. He also caught Walcott with one of his best combos and KO'ed him in their second fight.
Since Marciano doesn't really count for he beat Louis, we'll give that spot to Billy Conn. Conn is known as one of the greatest light-heavyweights of all-time, and for a heavyweight to try and catch a light-heavyweight is a hard thing. Louis KO'ed Conn twice.