GGG v Hagler is a good one to imagine – Mr unstoppable force, please meet Mr immovable object.
Think Hagler would be too accomplished for GGG. Hearns was the magician of that era but the Marvelous one was a skilled technician. That’s to go along with a chin made of depleted uranium and generally being harder than a coffin nail.
That time was just different gravy, though, compared to today’s middleweight scene. I suspect we’ll never know if GGG is an all time great as there’s just not enough quality around for him to prove it [and he got a late start with his pro career]. Moving up seems like a stretch just looking at him, doesn’t seem to have the frame for it, but maybe he could do something unexpected there at supermiddle.