If PBF and PacMan never fight, by my estimation, they would be the 2 greatest fighters to have never fought each other while a fight was doable. They are both sure-fire HOFers peaking at the same weight-class and same time, arriving back-to-back 1 and 2 on virtually everyone’s 1b-4-1b list. I don’t know of a comparable situation in boxing history.
But what comes close? Among those who could’ve reasonably fought each other, who else qualifies? This is what I have off the top of my head:
1. JCChavez vs Azumah: Both HOFers who peaked during the same era at similar weights.
2. SRL vs Aaron Pryor: A little iffy in terms of do-ability since Pryor peaked just as SRL retired early, but they are both HOFers of the same era and similar weight.
3.  

uran vs Aaron Pryor: I never thought about this possibility before now, but If Duran didn’t get so fat so early, I don’t see why this couldn’t have happened at welter in the early 80’s, maybe right after the Argeullo fight. Both HOFers, of course.
4.  

uran vs Argeullo: Same as above. Too bad Duran had to become a puff-ball. Both HOFers.
5. Foreman vs Tyson: Both HOFers though Foreman was on his 2nd act when the fight was doable. Tyson was in unbeatable territory at the time and Foreman was considered a joke, but we know in hindsight that Tyson was never really that and Foreman was no joke, even at his advanced age. Foreman never went down in his comeback and could only be out-boxed. And Tyson couldn’t fight going backwards so the style matchup would’ve favored Big George. And it would’ve been a clash of titanic power. I recall reading that the Tyson camp knew Big George was more formidable than met the eye, and did not want to risk a fight unless the reward was there. But it never materialized as Tyson descended into prison.
6. Sweet Pea vs Taylor: Meldrick had the fastest hands I’ve ever seen. There was no way to KO a prime Sweat Pea, so power doesn’t matter. But Speed? Maybe? If anyone could confound Sweet Pea, the PBF of his time, it was Meldrick. But his career all but ended after JCC, so there was only a narrow timeframe.
7. Mosley vs Trinidad: not sure why this never happened. They were both welters for a long time and The Ring had them ranked 1 an 2 on the 2000 1b-4-1b list.
8. Lewis vs Bowe: The two big Heavys of the same era who could both box
9. Foreman vs Shavers: 2 biggest punchers of all time. Wouldn’t have been pretty, but you know you have to watch.
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