¿Is Golovkin in his prime an ATG MW (H2H)?

Head to head, do what you like.

Head to head is so complex and so much more complex than people think. GGG is not a large middleweight for his era but he would have a singificant size advantage over someone like Dick Tiger and a huge size advantage over someone like Stanley Ketchel. Speaking of Ketchel, the rules between his era and GGG's are just different - they're just so different. Could Ketchel possibly adapt to these massive gloves, unable to weave punches and find corners and nooks for his devastating power? Could GGG possibly last 25 rounds and keep his temper being brutally fouled by naked laces? We don't know, it's speculation way beyond how styles and attributes might mesh (and be honest, we don't have a deep understanding of even that).

The reason people who really understand boxing are reluctant to go overboard in lauding the head to head aspect in all time standing is that it has so little meaning. How can we measure A tiny guy like Hugo Kelly against a relative monster like Hopkins? How can it possibly be imagined that Hopkins might weigh in at 158lbs ringside (or near it) as Kelly did for his world title matches and perform at his maximum? How can we know how good Canelo might be in the 20th round of a fight with indeterminate timekeeping and an actual barn wall for the fourth rope? We don't know. We don't have the first clue.

What we can say about GGG is that he was a wonderful fighter with many great attributes who would be competitive fighter in his weight range from all of history, whether that's the light-heavyweights of the early twentieth century or the super-middles of the 1980s or the middleweights fighting today. But the words "all time middleweight head to head" mean nothing to me.

What you might try to do is try to compare overall skillsets versus the era's rulesets for all middleweights who are up there, but it is a task few I know of would have the patience for on the forum today.

All that said -it's fun probably to think about for a lot of people, so have at it, just another thought.