I wouldn't pick anyone over Hopkins with certainty. He is rough enough to maul his way through Robinson. (If blown-up welterweight and less-skilled Carmen Basilio could do it, and if brawling, straight-forward Gene Fullmer could do it, Hopkins sure as hell could.) He is technically gifted enough to take it to Hagler and Monzon, too. Hagler had trouble with tough guys. People forget, but Vito Antuofermo gave him trouble, as did Roberto Duran. Hagler actually had to come from behind and fight like hell to salvage that one. Hagler lost some decisions early in his career to some Philadelphia tough guys, and Hopkins is the best Philadelphia tough guy since Frazier. I haven't seen enough of Monzon to make an assessment, but from what I have seen of Hopkins I know that he knows how to fight, is versatile, and is never in less-than-perfect condition. Hopkins may or may not be the # 1 guy of all time at middleweight, but he definitely is in the final running and deserves to have his name right up there next to the other top middleweight legends.
Marciano's heart, desire to win, and punching power are given their just due. I don't think he was the best Heavy ever, though. His 49-0 record is what gives him his mythical status. If he'd lost that decision against LaStarza, it would have made all the difference in the history books and top-ten ratings. Marciano deserves his kudos for being able to gut out victories over Charles (1st fight) and Walcott (1st fight) and then killing them in the rematches. A prime Louis probably would have shredded him, however, and Larry Holmes or Ali would have had their way with him for 15 rounds. IMHO.
I think Joe Louis underestimated himself. Louis had the skill, speed of hand, and power in his prime to mess Marciano up badly. Joe murdered guys who came towards him. He devastated them. If Jersey Joe's left hook could floor Marciano and cut him up, and if Archie Moore's right hand could drop him and give him some trouble, and if Roland LaStarza could give him issues, imagine what the prime fists of combination-punching Louis could do. The fight would be stopped around four or five with Martciano's face a gory mess. Marciano could throw a one-punch kayo, but against top guys he had to grind them down a bit first. I see Louis doing most of the early rounds grinding as he chops Rocky's face to shreds with his combos and jab.
If you take a look at the history of boxing and think about it, it's plain to see that the USA has produced virtually all of the best fighters in the upper weight divisions and Mexico has produced many of the best fighters in the lower weights. I'd say Mexico is the place to be for a boxing fan; it's still a tremendously popular sport down there and only the best make it to the top. Then they come up here and fight for the big bucks.
Puerto Rico? Good fighters, but alas:
Hopkins squashed Trinidad.
Chavez dissected Camacho.
Sanchez mopped the ring with Gomez.
The best Puerto Rican fighter was probably Carlos Ortiz. I can't think of many other Puerto Rican legends.