Rocky Marciano, for whatever it's worth, was generally too small to even be fighting in today's heavyweight division. He had a sort of an iron chin and could take horrific punishment (for the day), body fat in negative numbers, a very good punch with his right hand, good hooks with both hands, and an unusual knack for throwing the hardest punch he could LAND rather than the hardest punch he could throw, most of the time at least. He didn't have anything remotely like the defensive skills or hand speed which Joe Louis had in his hayday and I'd rate Louis the better puncher of the two, both in their primes. He knocked Joe Louis out in a fight way beyond Louis' prime which Louis had been forced to take in an abortive age-40 comeback effort to try to settle debts to the IRS which were basically illegitimate in many people's views.
But in his professional career he [Roy Jones] won titles in the middleweight, super middleweight, light heavyweight and heavyweight divisions. He is the ONLY fighter in history to start his career as a junior middleweight, and win a title as a heavyweight.
Jones and Duran had the same problem, i.e. at some point they'd scared everybody close to their own sizes into the woodworks and had to start trying to beat up progressively larger people to earn money at it. At that point, a career change of some sort is in order.
watching the thrilla in manilla on ESPN2 and remembering when this sport was king...