I've often thought they had the similarities, tough chins, had a few wars, loved to rough it up and maybe, just maybe had at least one other winning big fight in em. As far as the best, I'll say Marvin but it's close.
If it's "not P4P" I can't see any argument for Hagler. Marciano was clearly better. Bigger, stronger, harder hitter. Beat better, bigger, stronger opposition.
By not P4P I mean like not who ranks higher P4P. I don't mean who accomplished more, which is also Hagler, I mean who was better at what they did in the ring.