Well this is interesting. In a recent Reddit AMA, famed MMA fighter Ronda Rousey claimed that she would be able to handle Floyd Mayweather in a no-holds-barred fight. She did make sure to specify that he could take her in a pure boxing match:
Do I think I would legit beat Mayweather in a ruleless fight? Floyd is one of the best boxers of all time. He would definitely beat me in a boxing match. I unfortunately don’t get into “matches”. I fight for a living. In a no rules fight, I believe I can beat anyone on this planet.
That’s a fight I’m sure the promoters would love to get their hands on, but I sincerely doubt we’ll ever get to see.