George Foreman by a mile. The guy had two separate champion career runs with a ten year layoff in the middle. He also had a chin made out of something that fell from outer space and he was competitive all the way into his mid 40s.
I don't think Beterbiev is just a product of hard work. He's obviously naturally powerful and strong and he comes from as region and very masculine dominant culture with a very rich tradition of fighting and producing world class and elite combat sports athletes.
Yep, we need to throw out the roids, which are never a known for sure, to have any real conversation on this. Manny and Canelo +++ too many to count are and were straight up roid boys. You used to see some speeding in the 60s-70s even. Which gives the older fighters the nod to me in this foolishness of x vs. y. MANY today are juiced and bigger and 'stronger' than they ever should have been.
Tyson is the biggest genetic freak. Someone put it best, the speed of a man half his size and the power of a man twice his size.