GGG would easily go better than the rest of them. He has enough assets to make Hagler earn an honest nights work.
GGG is probably the best middle weight he’d have fought right? Hamsho, Mugabi, Roldan, Vito, Minter and Sibbo etc were good fighters but I always felt like Hagler didn’t get to test himself the way he could’ve if he fought in the 30s-60s I see Hagler on a similar level as Griffith in skills, really good, not much between them in that regard both great skilful boxers but one more proven, more tested - I’d say he should beat GGG (who is a little bigger) but it’d be probably his hardest fight and if he fought him from 84-87 I’d expect an upset - GGG actually reminds me of Hagler in that he was a bit untested for an ATG but obviously even more so, shame how his career panned out he was so close to recognition but he didn’t get the shots I suppose? Or the business people had other plans?
Best chance would be Triple G. If he's prime. But his strength as a boxer is what Hagler feasted on. He ain't beating Hagler prime vs prime. Other 3 are Canon fodder.
GGG is clearly the dangerous one. Hagler wins, but it won't be a walk in the park. Martinez and Janibek are good but outclassed; they are one more paycheck for Marvin. Duckelo is never ever going to fight Marvin, not even in a fantasy match up.