Monzon on points*. Canelo got painted by Golovkin's jab and Monzon had one of the best ever. I think ginger will see the final bell because he's built out of armourplate, but I'm pretty sure he loses. *Assuming impartial judges and not the Vegas specials Canelo keeps in his footlocker.
Outside of Vegas, or having the event a few weeks after Monzon had just fought and without a catch weight clause, Monzon jabs Canelo repeatedly which in the later rounds gives way to a few combinations in a points victory for Monzon. Mayweather jr. put on a clinic in the manner he stuttered his jabs against Canelo. GGG also bested him with the jab.
Monzon takes Canelo apart. The ability to control distance, the jab, the sneaky, well-timed straight rights, etc. Top-tier ATG middleweight vs. a really good one.
Monzon is the best MW in history imo, unlike others Monzon would've clearly beaten Canelo with no doubts.
Monzon smokes his ass like he smoked 100 cigs a day when he was out of training (only 50 during camp) Smokes his ass like a prime GGG would've