Peak Ali and then peak Holmes. Norton pushed him hard but he was hell for boxers without atomic power.
Tunney would be the hardest to actually beat, but Ali's star-power, especially in the late 1970s, would make him the hardest to beat on the cards. Ask Ken Norton and Jimmy Young about that.
We all know Marciano gets cut to ribbons by any black boxer that has a jab.. The poor little Italian just couldn't take it.
I listed Tunney and Wlad as my choices, but several others on that list would be good picks. I also agree with those who mentioned swarmers like Frazier and Marciano as being difficult to outpoint.
Mildenberger didn't give Ali fits and hardly won a round in that fight. True, he was a bit more difficult for Ali than most other opponents around that time, but it was by no means a close fight. It starts to annoy me that Ali's difficulties against some of his opponents is blown way out of proportion. "He really lost against D. Jones, he really lost his second fights with both Norton and Frazier, he really lost against Shavers, Foster mauled his face, Lyle would have won if the ref hadn't stepped in even though he shouldn't have etc, etc". But I agree with you that D. Jones hardly would have troubled the Klitshckos. Maybe they are the hardets ones to outpoint since no one ever has been even close to do it as far as I know. But that point can be made without overexaggerating the blotches on Ali's record.
He only had one fight at heavyweight, put him in with peak Holmes and co and he might be hampered to say the least.
I pick prime Ali. Yes, he gave up rounds against Mildenberger, Cooper, and Chuvalo, but those fights were only just competitive. One Cooper knocked down Ali, he was stopped in the next round. Ali was known to toy with his opponents, even letting Chuvalo repeatidly him in the body several times. My second pick would be Wladimir Klitschko, you'd do well to hit him square on the chin and not knock him out. That's why he's difficult to outpoint.
It seems that that power and size factor into this equation a bit more than meets the eye. Ali could not keep Chavalo off him, but bigger hitters could.