I don't think there's much to choose from per actual power, but Hearns was far more effective and lethal therefore in his delivery of his power shots.
IMO Graziano has more pure power but Hearns was faster and more acurate which made him more effective. You could say Hearns had more effective power.
Rocky had the greatest "one, two" combo... His left hand holding you under the chin, followed by a right hand grenade... Very effective, I'd say. Not Marquess of Queensbury though....