Who stand the best chance against a prime Ali?

Nobody who isn't a front foot aggressive attacker is going to beat Ali. That rules out Wlad and especially Vitali. Joe Bugner showed how those guys would look

 

That's always my dark horse in fantasy fights. High chance of him blowing just about anyone out in a round or two if they have zero foreknowledge. Nobody, NOBODY would expect his speed and combos with that short stocky build. By the time they appreciated it, they'd be on the canvas, and good luck once Tyson has you hurt.

 

Pressure fighters who fight aggressively (Swarmers, if you prefer calling them that), and are at least close to Muhammad Ali's level, have good chances of pulling out wins against him, in several tries. I am thinking, of course, about fighters such as Sam Langford, Rocky Marciano, and Joe Frazier. For other styles, good Boxer/Punchers, such as Evander Holyfield and Lennox Lewis could get wins, again in several tries. As for the Boxer/Punchers, I think Joe Louis could get him six out of ten times. I rank Joe Louis #1 all-time, and Muhammad Ali #2.

 

When you post outrageous things and nobody responds it's more likely than not most posters have you on ignore. That's a universal law of the internet.

 

It's the L boys for me, Louis, Liston and Lewis.

While I see Ali still beating Sonny, the peak version of Liston would be far more competitive. An interesting fight for sure.

 

It would only be the swarmers, guys like Frazier and Norton who gave him problems as they were in his face and negated a lot of his greatest strength which was movement and speed.
Guys like Marciano, Langford, Holyfield to an extent when he was really busy with his punches. I am not saying they win of course, I think Ali at his peak still wins, but they would certainly give him a fight.