He's also down to box Khabib Nurmagomedov -- but again, he wants $300 million.

But, if you're hoping to see Floyd take on a legit contender, don't hold your breath -- Mayweather says he's only taking easy fights from here on out.

TBE spelled it out on Shannon Sharpe's "Club Shay Shay" podcast -- where he explained why his longterm mental health is more important than a paycheck ... which is why he'll only take fights he knows he can easily win.

"Conor McGregor can't punch," Mayweather said ... "He can't punch."

Floyd says he remembers taking a shot from Conor during their 2017 fight and thinking to himself, "Man, this dude punch so soft. It's cool."

Mayweather says he looks at fighters like his uncle, Roger Mayweather, and Muhammad Ali as cautionary tales ... saying, "Your health is your wealth."

Floyd says he's still training and working out -- and isn't far from his fighting weight ... but it doesn't sound like he has anything lined up just yet.

Of course, he's been mulling fights with Conor and Khabib for years but nothing has materialized.

There's more ... Mayweather also explained why he should be considered the greatest boxer of all time -- ahead of legends like Muhammad Ali and Rocky Marciano.

"Is Ali the same fighter that got beat by a guy with 7 fights? Leon Spinks beat Muhammad Ali when he only had 7 fights," Mayweather said.

"If we really tell the truth, what's the heavyweight guy that Ali fought 3 times? Ken Norton really beat Ali all 3 times. Go back and look at the fights."

Mayweather says he loves Ali ... but he's "just telling the truth."

He also spoke about Muhammad Ali's activism outside the ring -- and claims he would have done the same thing if he were around in the '60s.

"Ali will always be a pioneer because of what he stood for in that era. And, I commend him for that."

Ali famously refused to be drafted to fight in the Vietnam war and was subsequently stripped of his boxing titles.