Leonard took the Duran fight when Duran was at an absolute beast. No jokes there. He was hungry and in peak form. Credit to Leonard for taking such a fight that could make people look down in him today in Sherdog forums. Mayweather didn't fight certain guys in their primes. Pacquiao, for instance, was 5-6 years too late, and when he did, well, we saw what happened - a one sided but a clutch and hold snore-fest. Still a win, sure, no argument, but people can criticize those circumstances as/more easily than Leonard who fought at guy at his best, and then won in dominant fashion in the rematch.
Ezzard Charles is being seriously underappreciated here. I will say that his LHW run gets underappreciated. Different afflictions, sure, but Charles was really remarkably skilled. If you watch him, you'll see that a lot of what he does, bigger guys don't normally do. I'm not necessarily arguing with you here, but I'm more coming to his defense. He fought in one of the best eras of boxing where the gyms were packed, the trainers knew so much, and even average fighters then were actually really good and would probably be upper crust today. No, that's not because I'm smitten by black and white footage, that's just the facts. More minds, people, infrastructure invested in something, the more talent you'll see.
The sport was not pro wrestling, and ignoring guys like Gans and Langford is just a disservice born out of ignorance. Having close fights is not a red flag, it's indicative of the times. Nobody boxes today, thus, the talent pool is not as deep.
No one is saying that Floyd isn't a really good fighter, because I agree that he is, but you are riding on him hard here. Somewhere in the low end of the top 15-20 there could be a case made.
When you are fighting guys that are at their best, and you lose to them, you shouldn't be penalized for that. You should be appreciated for reaching for greatness. Ray Robinson had more than a few losses, but he's better than Floyd. He did more. He went from WW to LHW and fought anybody and there are very few fighters that have made a successful move from WW to MW (and that's due to the quality of the opposition - SRR was just special. A guy fighting a surging, dangerous fighter, at his absolute best shouldn't be penalized for losing. If that fighters waits 6 years and then fights him and beats him, it's an admission of his own fear and insecurities about fighting that fighter.
Sometimes you make some good observations, but you are coming on too strong here in your Mayweather love and not being as objective as you ought to be (which can be hard, I'm sure).