It isn't hard at all, it's Hagler's. The fact that Marvin usually fired back immediately and traded blows when taking a big shot only adds to that, whereas Ali would follow absorbing something similar with a clinch (nothing wrong with that, but generally not what Hagler would do). The ****er didn't even grimace.
If you take away the Ali fight, Big George fought... Chuvalo Norton Frazier Holyfield Morrison Moorer Briggs