His ability take the lead against fighters in their prime would have been put to the test against guys like Mosley or DLH. I'm not sure Mayweather had the offense to beat those guys in their prime. And the most likely scenario is he gets outworked.
Slightly, but relevant. Mayweather never displayed the ability to outwork anybody elite in their prime. Ever.
I am saying that's speculative though. I think your actual career is more important as to weather you could beat said fighter on a different day or not. Anyway I don't think Mosley was really competitive at all he got mostly shut out other than that one round were he rocked Floyd. With OLDH you could say he at least was competitive so maybe that can influence some thoughts on what if OLDH was younger considering OLDH's point of his career it was impressive he challenged Floyd tbf.
Inoue, in my opinion, is in 1st place, followed by Pacman and Mayweather. Mayweather has had countless close fights. Paquiao has lost fights and also had plenty of close fights. Inoue has no losses or close fights, yes, including the first fight with Donaire. Inoue is the best boxer of the 21st century.
Mayweather has been the biggest fighter in the sport this century. Close contenders Wlad Crawford Alvarez GGG Ward Usyk Calzaghe DLH Lewis
Many good ones: Usyk, Pacquiao, Crawford, Calzaghe, Inoue. I rate Mayweather lower because: 1) he always had home advantage, he never had the courage to fight on the road 2) he received assistance from referees (Cortez) and judges (Castillo I). 3) he was VERY carefully matched, waiting until the better fighters had been KOed and were faded (Oscar, Pacquiao, Cotto, etc.). Mayweather avoided boxers in their prime. 4) he fought boxers from smaller weight divisions (Hatton, Pacquiao, Marquez, Guerrero) while never fighting anyone one division higher (Trinidad, Forrest, Tszyu).