Ricardo Lopez at 105 Wilfredo Gomez at 122 Pernell Whitaker at 135 Ray Robinson at 147 Roy Jones jr at 168
Prime Roy Jones at 168 would be strongly favored to beat Calzaghe. Prime Michael Nunn would have a shot. Prime James Toney too.
Give me a break. If Castillo could beat Floyd at 135, then Chavez would cave his ribs in at 130. I don't think there has ever been a fighter who could beat every other fighter to fight at a given weight class, but I'd say probably the closest thing to it would be Roy Jones Jr at 168lbs. He was a physical phenomenon. But as I say, I wouldn't back him to be unbeatable there, because he didn't fight anyone of any real quality at that weight other than Toney, so there isn't enough evidence to suggest Jones was unbeatable there, that's just my own impression from watching him there. Obviously Ray Robinson at 147 is a shout for people who have seen a lot of him at welterweight (I haven't), but 147 is such a deep, deep division that I wouldn't say he was unbeatable there, there are too many great fighters. Same with 135, which rules out Duran, Whitaker, Benny Leonard, etc.
basically all the answers to this topic is: who was a BEAST at a certain weight class... but the fact is is that nobody is invincible. there will always be a fighter who throws another for a loop, that's the good thing about boxing. lamotta and robinson, pep and saddler, blah blah blah I'll say this though, in their primes Benny Leonard(LW), Pep(FW), Louis(HW), Arguello(SFW), SRR(WW), Armstrong(FW-WW) hauled unreasonable amounts of ass, destroying competition.
RJJ at SMW, Sal Sanchez at FW, Gomez at JFW, Robinson at WW, Floyd at SFW, Chavez at LW, Monzon at MW, Finito Lopez at SW.
-Ricardo Lopez at 105 -Wilfredo Gomez at 122 -Evander Holyfield at Cruiserweight (provided we aren't considering former Heavyweights who routinely weighed in below the 200 pound mark)