Mercer. Guy had a titanium jaw, one punch power, very fast one punch speed, accuracy, and had more skill. Mercer would KO Shavers in the first 3.
Mercer was skilled, had heart, and a great chin. Shavers would be one of the few fighters that could hurt him but I don't see him KO'ing Ray. Not before Ray did anyways. Mercer TKO 6.
I agree that Mercer would win and that his chin would help him prevail but Mercer wasn't all that fast IMO. He wasn't all that hard to hit either. He had good power but not the kind of power Shavers had. Morrison was faster than Mercer and landed almost at will vs. Mercer for 3 1/2 rounds. Morrison had more power than Mercer but it didn't matter. Like Shavers, Morrison's chin was not as good as Mercer's. Mercer didn't do anything great IMO but he had a pretty decent jab and the basic fundamentals down. What made him hard to beat was his toughness. He would eventually stop Shavers IMO.
The 90's could have been the best era of HW boxing but only Holyfield fought the other top HWs... so the 70's remains tops. Tyson, Holyfield, Bowe, Lewis, Foreman, Holmes, Moorer, Ibeabuchi, Byrd, Tua, Rahman, Mercer, Morrison, and others.
No doubt Mercer wins this one. It would be an entertaining fight but he eventually stops Shavers in the middle rounds.