I think in a hypothetical Haggler vs Monzon matchup you'd have to lean towards Marvin for the following reasons...
#1 He's a southpaw, the jab (no matter how great) would have not been the wise punch to keep pushing out there.

#2 Haggler's work rate, he worked every second of every round

#3 Haggler's chin, if he could take flush shots from Tommy Hearns he could take shots from anyone

#4 Distance, Haggler would smother Monzon. Monzon was a fighter who liked to get lots of leverage on his punches and it's hard to transfer that weight from the back foot to the front foot when someone is right in your chest.

I just think if you're going to beat Marvin Haggler you'd either need big time Bob Foster type power or loads of speed and either way you'd need great stamina and solid defense.