Monzon was undefeated in his last 81 fights, had power in both hands and possessed one of the most lethal right hands in boxing history. 59 Ko's! His size, power and boxing ability (Monzon was no slugger) would see him win a late round stoppage or a UD over a frustrated Hagler.
I think Monzon's low proifile in the US goes against him in discussions like this, overall i think you will find that Monzon is ranked the number one Middleweight of all time. What a fight it would have been though, it was talked about and nearly came off. Monzon was 36 when he retired.