1965-67 Muhammad Ali defends his title against the following modern day heavyweights

Tua and Morrison could maybe score a surprising knockdown out of nowhere as both had good left hooks which Ali was susceptible to.

But outside of that Tua would lose a wide UD Ali would dance rings around him.

Morrison doesn't have the chin or the gas tank and would get stopped between 8-10 rounds.

Ali in poor health and totally shot had a semi competitive fight vs Berbick so no reason to think a prime Ali wouldn't win convincingly vs Berbick. But I'll give Berbick some credit unless you were a puncher and caught him cold he was normally pretty durable. Berbick went 15 rounds against a peak Holmes so I think Berbick could go the distance with Ali aswell.

Mercer made Damiani look like Ali so no reason to think Ali wouldn't also dance rings around the plodding Mercer and win a wide UD.

Bonecrusher is a crude slugger but pretty durable he loses convincingly but I think he goes the distance with Ali.

 

Morrison - Ali via stoppage after maybe taking a flash knockdown.
Berbick - Trevor lasts distance but Ali scores a shutout,winning at least twelve rounds.
Mercer - as Berbick fight.
Tua - out of them all Tua gives Muhammad the most competitive fight. Muhammad still wins by a clear margin over the distance though.
Smith - Bonecrusher is durable enough to last the distance but loses virtually every round. But if a post-prime Holmes could cut him then there's an extremely good chance that prime Muhammad could. Therefore the bout is stopped.

 

I doubt any heavyweight in history could have beaten Ali during those years. Joe Louis was too stationary. I'd give Ezzard Charles an outside chance at an upset. Maybe 9-to-5. Or possibly Gene Tunney could figure something out. Edit: I just thought of another, possibly the most formidable opponent for prime Ali: The best version of Larry Holmes.