We need to be a bit less nostalgic here.
Hagler was not a massive puncher, solid shots but not a one punch KO.
I would like to have seen Julian Jackson land some shots and see how Hagler would of dealt with those bombs or GGG is just as dogged as Hagler.
MW Roy Jones would beat Hagler but wouldn't slug with him.
Funnily enough Sugar Ray Robinson, although known for being a boxer I've seen him slug it out really well.
If its a pure slug fest I'm going to go with ..........
GGG, Toney, Rubin Carter as contenders
Carter was like an early version of Hagler