Jersey Joe Walcott actually made a pretty clean sweep of the division before the first Louis fight. Just thought that you ought to know!
Larry never beat a reigning champion nor beat the 1/2 when 1/2. He is not fitting the criteria. The best one is Jersey Joe for me. He beat almost everyone in the top 10.
"Almost everyone" believed Muhammad Ali was the champion for the third time when he defeated Leon Spinks.
Nobody at heavyweight "cleaned out" the division before becoming champion. Even including after becoming champion you'll be left with maybe two candidates, Joe Louis and Muhammad Ali.
Buster Douglas is the obvious proof that Tyson did NOT clean out the division. Douglas was a ranked contender before Tyson was. Tyson lost to him the first and only time they fought. You can't "clean out" a division and then lose to a guy who was hanging around the rankings all the time. That makes no sense. But to be fair to Douglas he may not have been as "big" a "name" as Witherspoon but his credentials during Tyson's reign were just as good and perhaps better.
Joe Brown at lightweight. Duran at lightweight, kinda, in the sense that he four times beat the best lightweight in the world excepting himself.
Never met the best fighter in the world excepting himself. No danger that's a clear out. But tbf, clearing out a division is really, really rare. There are even arguments against Brown and Duran.
Dempsey was seen as the underdog against Fulton and an unknown a few months before the bout. Fulton was seen as a top contender in 1917 and Willard ducked him.