What pre-Louis heavyweight has the best shot at beating prime Ali?

None of them, but I guess Dempsey for being a swarmer and puncher.

 

Probably Jack Dempsey

 

To expand on my answer and to answer the second question, firstly, Gene Tunney had one of the best jabs in the history of the heavyweight division. Tunney was a meticulous planner and had the ability to stick to his plans with extreme focus. He would be smart enough to throw his jab simultaneously as Norton did against Ali, which would neutralize the jab of Muhammad. He also had a very underrated left hook, which could cause damage to Ali in the course of the fight.

Tunney showed that he was a very solid body puncher in some of his earlier fights. Punching to the body is key to slowing Ali down, even though Ali's downstairs was made of steel.

Tunney was a first-class counterpuncher and liked to draw leads to counter. Countering someone as fast as Ali wouldn't be easy though.

My conclusion is that Tunney could pose a threat to Ali, based on the reasons listed above.

Dempsey's case is much simpler. As we know, Dempsey had a great left hook and was very aggressive with his pressure. The combination of those two things would trouble Ali. I am uncertain if Dempsey could catch a prime Ali, who could still fight for every inch if he needed to as seen in his post-exile career.

The ruleset would have a large impact on these matchups.

 

The Dempsey of the Willard fight...you don't beat that Dempsey with a left jab!!!

 

He was active for more than 5 years before Louis hit the scene.

 

Jack Dempsey would have the BEST chance but I still see Muhammad winning in an exciting bout.

 

Dempsey has no chance vs Ali just watch Cus D'amato's breakdown of the fight when hes with Ali.

I believe he said it was the way Dempsey leaned forward that Ali would pepper with him with jabs and Dempsey would never be able to get in punching range.

 

Dempsey by far.

 

The "Galveston Giant" was six feet tall.

Nobody who walks out and the first thing they do is grab is going to beat Ali.

So that eliminates 99 percent of them.

Gene Tunney would have the best chance because he could jab with Ali. But, he wasn't physically strong enough, IMO.

Ali was a pretty strong guy once he matured.