Well what criteria are we using? If it's his undefeated record, there are actually multiple other boxers who can challenge that claim such as Lopez or Mayweather. If it's clearing out his division, plenty of boxers have done that as well such as Louis and Ali who definitely had better competition too. They also have him beat in terms of beating ranked contenders. Robinson had several hall of famers. Rocky would be lucky to get placed top 15 for this category and he retired fairly early too. If it's showing dominance of an era and crushing the majority of his opponents with brutal KOs/stoppages, you have the Klitschkos, wilder, Julian Jackson, Hagler, etc. If it's skill, there are dozens of boxers better than him. Even if we limit the criteria to just pressure fighters, you could argue Frazier and Dempsey were his superiors, let alone lower division guys such as Armstrong or Chavez. If it's h2h, good luck convincing anyone who isn't a die hard Rocky fan he's the best. I don't see how anyone could possibly make a convincing argument.
How do you improve on perfection? Fought 49 times and won 49 times. Defended against the #1 contender as champion, with one sole exception, where he defended against eh #2. We can't ask much more from him, based on what was possible at the time. That is about as far as the argument goes.
Presumably you already know. That is not a devastating argument, if we are debating his place in all time top ten heavyweights. It might be a strong argument, when you try to insert him into the p4p argument!
I don't think you can make a solid case. His only argument is his clean 49-0 record, but he didn't face as good competition as Louis or Ali, he didn't fight as many fights and he spent almost half of his career on fighting nobodies. Rocky is a monstrous P4P fighter and clearly a top 10 HW ever, but he has no case for GOAT. He shouldn't be ashamed, only two people truly have.
Retired completely undefeated, champion. Not sure he actually ducked anybody worth too much, though I imagine there are arguments for that. Hard to leave him out of the top 10 imo.
Lol the only people who think Rocky Marciano is a heavyweight period by modern standards are people who don't understand weight classes. Marciano couldn't even beat most middleweights with how unskilled he was. I wouldn't part Marciano in the to 25 thousand heavyweights of all time. I see no way Marciano could beat any half way decent cruiser weight by modern standards. Could we stop it with these Marciano threads. The guy wasn't even good for this time frame. He just fought a bunch of bums, old guys, and mob controlled guys.
Anyway, about the actual thread question, you'd have to jury rig your standards for greatness and the way you evaluate some issues: * Brutally counting losses against fighters (rather like modern fans do...) * Accepting the now-less-popular thesis that once you get above a certain magic number in the 185/190 pound range, you're big enough for anybody. * Accepting the Arc of Boxing argument, but weakening it somewhat to avoid the obvious issue that the 50s weren't great even according to its standards. * A very subjective assessment of Marciano and his opponents' skills on film. Argue on technical grounds that Walcott, Charles, Moore, etc. were technical wizards, and place a lot of weight on beating boxing technicians in your greatness calculations. * Play around with figuring out his prime, since he's a small swarmer who would be expected to suffer from high attrition. Calculate it so that even some of his good wins are way past his prime, and his early trouble with Walcott is before he really hits his stride. Not too different from the approach used with Tyson. I'm sure there are more.