Which Heavyweight that Louis didn't face could have given him the most trouble?

Some other less obvious names:

Maxie Rosenbloom

Melio Bettina

Tiger Jack Fox

There was pleanty of trouble in that era if you knew where to look for it!

Of the fighters you listed, I would be inclined to name Murray with Tomphson coming second.

 

Former southpaw LHW king Bettina would have posed some interesting issues for Joe, and Melio the Mighty would have been a possible challenger during his wartime service. Did any heavyweight champion even defend against a southpaw before Ali took on Mildenberger and Dunn? (Historians on the board, can you identify anybody predating Mildenberger?)

In August 1940, Bettina rebounded from his decision loss to Joe's end of year challenger Al McCoy by winning a decision of his own over Louis sparring partner and training camp buddy Joe O'Gatty. I wonder if O'Gatty was simply on a mission to scout Melio out.

There have been allegations that Rosenbloom was avoided during the 1930s. At the end of his career, this veteran of nearly 300 bouts handed Nova his first defeat, and drew with Pastor before retiring with a sham knockout win over yet another Louis victim, Al Ettore. John Henry Lewis, Jack Kranz, and Lee Ramage were other late career conquests Maxie had in common with the Bomber. Rosenbloom would have been a reasonable challenger for Joe during the "Bum of the Month" streak, and would have afforded us our one opportunity to see Slapsy in action.

Walker discussed a proposed match between himself and Joe in his autobiography, but even the cocky Mick disclaimed that, "Even at my best, I know I couldn't have licked him."

 

What a great question!

Rosenbloom and Bettina give him the most trouble. Impossible for Louis to look good against either man.

Lee Q. and the Turkey are more dangerous and next in line. Louis could time the rushes of the squat Thompson and stop him. When Turkey got inside Joe's short jolts would do a number on him.

Discounting his inexplicably poor performance vs Walcott(which may have been a fix, IMO), the long, rangy and hard-hitting Murray is much more dangerous. His long arms and booming right would pose quite a risk. But Hatchetman Sheppard hurt and floored Murray, so Louis could eventually find the range and right punches to stop him. Plus Lee had Ray Arcel in his corner, which was the kiss of death when you were fighting Louis. It's a good heavyweight losing to a great one in a sensational battle that pushes Louis to display his best.

Lem Franklin's glass chin gets him KO'd early provided Louis doesn't make any mistakes (and he and Blackburn would make sure that he DIDN'T make any against a dangerous whacker like Lem). This would be a highlight reel KO for Joe Louis.

Bobo is big, strong and a hard hitter, but was stopped by lesser fighters than Joe. So after giving Louis some early problems he gets stopped within 8.

Tiger Jack's wide-open slugging plays right into Louis's hands and he is stopped early. Turkey Thompson stopped Elmer Ray, so Louis would too (provided, as with Franklin, he didn't get careless against so dangerous a whacker)