At first I thought about Muhammad Ali. But he had some very close calls and indeed lost to Joe Frazier while still being under age 30. It sort of lends itself to the strong possibility that he might have lost to someone like Joe Louis, especially in a trilogy... Not sure I can pick anyone with much confidence. I suppose Jeffries Would have a good chance of going undefeated against all his predecessors.... But then again, an aged Bob Fitzsimmons gave him all he could handle..
I already acknowledged Dempsey would be a serious threat early, but outside an early blitz punchers chance i just don't see him beating Louis at his best. Tunney never saw a guy who combined the evolved ring IQ, superb timing and technique plus good hand speed and powerful blows. The division had never seen anything like Louis, he was like a terminator sent from the future once he polished his style. Louis could match wits with Tunney and had more than enough height, reach, and power to counter Tunney's fancy feet and cageyness. Back then many big hitters were usually slow plodders or wild crawlers but Louis was a true boxer puncher and possibly the best combination puncher of all time. He was also one of the first heavies to consistently use a piston like jab throughout a round to set up his bombs (rather than swinging for the fences). I could only see Tunney pinpointing a green young Joe Louis 1/3. If the more experienced Louis could beat Walcott and Conn by brutal knockout out I don't see Tunney doing significantly better. He doesn't have a very deep resume at heavyweight anyway, Louis made 25 defenses against all comers. As for Jack Johnson, again I could only see him outhustling perhaps a young green Louis. For a so called defensive boxer, Johnson was KO'd quite a few times. I think he would frustrate Louis early with his excessive clinching and inside rough house tactics but eventually Louis would break him down. Standing right in front of the brown bomber attempting to harass and tire him out means you are leaving yourself open for brutal uppercuts and body shots. It would just be a matter of time.