Not the point under discussion here. I rank Hagler as greater than Calzaghe, mainly due to his having had a greater resume and career. But H-to-H, I believe Calzaghe takes it.
Hagler in his prime had better defense than Calzaghe. He had very effective parrying defense and head movement. The main question is how Hagler handles the jump in weight. Calzaghe would be an 175 lb. fighter back in Hagler's day, under different weigh-in rules. P4P, I pick Hagler by clear UD or maybe even he scores a late stoppage, but he might not handle the jump in weight.
You forgot about cuts too, Hagler was a bit of a bleeder. Also venue would be important, in the states JC would have no hope at all. In a neutral venue with neutral ref he would push Hagler close. If he can get Hagler to come after him he has a shot
Chin ? heart? Calzaghe has the amazing ability to recover from big head shots very quickly. Got up off the floor to stop Byron Mitchell in the same round. If he gets hurt he usually reacts by throwing as many punches as he can. Can change tactics mid fight ( kessler) With no s/mid at time ... Calzaghe by UD at 175
Is Calzaghe good at cuting fighters up? I know Calzaghe can throw a very good amount of punches, but can they tear?