Hagler by decision or late round tko. Good fight, back and forth but hagler eventually wears down monzon, swelling him up and perhaps even cutting him. Monzons slow feet would be a big handicap against a rampaging power puncher like hagler.
My guess is Hagler would switch to southpaw and try blocking Monzons Jab with his right hand and try and get under Monzon to work the body.This a 50/50 fight if ever I saw one.