The most obvious answer would be Canelo since he won the rematch and is entering into a higher dimension in the realm of boxing, but there is an 8-year difference between the two with Golovkin being 36, while Canelo is just fresh into entering his physical prime years as an athlete at age 28.
How Canelo's career will proceed in the next few years as a fighter in his 30s remains to be seen, but Golovkin was at an age where boxers would start seeing their athletic ability and reaction speeds wane. It's also the age range where fighters who once used to be in their primes start racking up losses to younger, hungrier fighters who are developing into their own athletic primes.
Canelo on the other hand, has the body frame not all that different in size from Floyd Mayweather, from the photos taken at the weigh-in. Both come into the scales looking shredded; it's just that Mayweather is always lean and pretty much shredded during his fights because he's one to train as if he was preparing for a competition all year long.
We're talking about a fighter who may be between 5'7 to 5'8 tall btw.
As for what this means? It means that when Canelo moved up to weight classes like MW, SMW, and LHW, he started adding muscle (along with some fat; he's not really shredded in his fights in higher weight classes) so he could gain enough wait to compete in those classes. Normally this tactic would be avoided by other fighters since they would be concerned about the extra added weight slowing them down + the natural side disadvantage that they would encounter from fighters at a higher weight class, but somehow, Canelo managed to use this "disadvantage", and turn it into an advantage instead; he retained most of his speed, agility, conditioning that he had as a fighter in a lower weight class.
This automatically makes him one of those fighters (particularly Pacquiao) that ignore the concept of potential disadvantages of moving up weight classes, just because they could, regardless of whether they get sent into obscurity after a pair of losses or not. Mikey Garcia attempted this tactic when he moved up to Welterweight to face Errol Spence Jr. and unfortunately, he lost a rather lopsided decision.
It is uncommon for a boxer to remain dominant even a couple years after their physical prime age, but gaining over 20lbs and moving up multiple weight classes so that he could take a fighter's belts from them?
It makes him an all-time great firmly in the ranks of fighters such as Sugar Ray Robinson, Pacquiao, Duran, etc.
Canelo may have a better resume than Roy Jones Jr. right after the John Ruiz at this point with the championship bouts in higher weight classes factored in, but this is debatable.