Who would have been Ali's toughest matchup among boxers he never fought?

I always considered him and Jersey Joe Walcott having an amazing match. Both extremely cagey in their own way, with Ali being bigger, but Walcott being better inside and outstanding at countering the jab with that fully loaded right hand or inside slip left hook.

 

Possible I guess.

 

Jeffries
Fitzsimmons
Sharkey (Tom, the good one)
Peter Maher

 

^ The 4 horizontal chumps of the bread line.

 

Ali did showed some vulnerability to a lefthook. Tyson had an outstanding hook. Overall more explosive than Joe but his pressure was not the steady kind Frazier applied.
In a long hard fight Frazier had a mental toughness Tyson never showed. Though it is the early rounds I would lowkey fear a little for Ali. Tyson for a time at his peak brought a speed and power that was not easy to prepare for.
As I said before in my own view Ali's route to beating Tyson would be an ugly one where he takes Mike into deep water and drowns him. It would depend on if Tyson knocked Ali down once or twice. If he did dominate early.

 

Buster Douglas, Wladimir Klitschko, Tony Tucker, Harry Wills, George Godfrey(II), Primo Carnera are tough matchups for him.