It can't be measured, but, my vote would have to go to the original enforcer, John Ferguson.
His destruction of 'Terrible Teddy Green' in the playoffs sparking the Habs to an upset was just one example of him making a difference with a fight. He didn't just fight, he went on the ice with the sole intent of winning through intimidation, but, when he fought he almost always seemed to make a difference.
Any sociology majors want a thesis idea? How about a statistical examination of the in-game effect of fights. We often hear the argument (by fighting proponents) that fights energize teams. It would be tough to research completely, but I'd love to see some good scientific evidence of it.