Hold up, @primo, who is "we?" I didn't see anyone arguing about Golovkin's ATG status, I saw you claim he isn't (Powerpuncher changed his mind and Ron is Ron) . You didn't give any reason why Hagler, Hopkins or anyone else rank above him, and now you're basically naming famous fighters and assuming they rank above, all the while only showing your own bias (personal opinion) and making my point (don't get hung up on this, it's not criticism).
Do you know who Hagler and Hopkins most famous fights were against? BOTH these all-time greats were considered "average" champions until facing famous welterweights at the end of their middleweight reigns. They both started underdog against welterweights!!! Hagler - 50 fights into his career - was the underdog against Alan Minter!!!
My whole point is - Individual fans don't get to decide who the greats are, the popular concenus decides. In 20 years the MAJORITY of fans and history books will do exactly what you've just done, list a bunch of famous fighters who shone brightly during their ERA, it won't matter that Golovkin defended his title against Vanes Martirosyan and Matt Macklin, just as it doesn't matter now that Hopkins defended against Robert Allen (50 times) and Carl Daniels, all that matters is they were long standing champions.
Golovkin's legacy isn't harmed by the controversial loss to Canelo, just as Hagler's controversial loss to Leonard doesn't harm him, it enhances the mystique (and GGG still has time to right it).
As for all-time great lists? I have history books of the HUNDRED greatest fighters of all time. Shit... back in the day old Britkid used to do his HUNDRED ATG middleweights every bank holiday on here