Jeffries in his prime? If you ask me only a handful of boxers could achieve that. Their names: Joe Louis Jack Johnson George Foreman Sonny Liston Jack Dempsey Rocky Marciano anyone else in my eyes doesn't really stand a chance.
No way for Shavers. He was very strong, but Jeffries was much too quick for Shavers. Add that to the fact that Shavers punched himself out early, and his "not-so-legendary" chin, he wouldn't be much of a fight. I see Shavers getting KO'd in the 4th or 5th during a trade of combos with Jeffries. And from all accounts I've heard, punches didn't really affect Jeffries. If there is one man in history who could emulate what Holmes did after gettin knocked down by Shavers, for me it would be Jeffries. Tyson has a better chance than Shavers. I think that Tysons intimidating swarming style would be negated right from the start by Jeffries ring generalship and footwork. He could glide across the ring while keeping a perfect fight stance, and I think he would take advantage of this skill most during a fight like this. After round 6 or 7 as Tyson's intensity would start to wear off, I think Jeffries would start really putting work in, and eventually he would go for the kill around round 8 or 9. That is my prediction if I had to call it.
Here is the problem. You cant compare what fighters of any era did, to fighters of any other era! In fact, it is near impossible to compare and predict what fighters of the same era are going to do against each other. And Video barely really helps. If this was not the case, we could all give up work and make our living betting on boxing events! All you can really is compare a fighters traits, relative to their own era. When you do this, not only would Fitz not have got creamed by Frazier and Ali, but he did not get creamed by the Frazier and Ali of his time! The equivalent of Ali was Jim Corbett. Like Ali, Corbett was considered by many as the great of All time, Like Ali, he was faster than anyone had ever seen before, he beat a fighter many considered an ATG in Sullivan (like Liston) and he generally floated like a butterfly and stung like a bee with a larger than life persona. Unlike Foster, Fitz knocked him out! Fitz had his Frazier too. Tom Sharkey had a ton of heart and courage and was one of the best fighters of all time when he met Fitzsimmons. His fights with Jeffries and Corbett showed his abilities. Arguably he did better against these two than Frazier did against Ali and Foreman. Either way, Fitz beat him twice, by KO. Now compare these results to what Foster did against Ali and Frazier! Or compare some of the guys who took Foster the distance to the guys who took Fitz the distance (not counting 4 rounders). Compar the fighters that went more than say 6 rounds against both fighters. Compare the amount and quality of the best fightes who went less than 4 rounds against both fighters. No matter which way you do it, Fitz comes out on top. Foster may very well have hit as hard as fitz. But if he did, he had a lot more difficulties in landing and a lot more off nights. Fitz' power was more an equivalent of prime Tyson than Bob Foster. Back to topic, I think that Prime Bob Foster has a very good chance of knocking out Jeffries. No one is incapable of getting knocked out (not Ali, Oliver McCall, or anyone else). And Fitz does have a chance to do the same to jeffries.
Given the level of opposition that Jeffries faced early in his career it is incredible that he was never on the canvas untill his fight with Jack Johnson. You can say what you like about the size of Jeffries oponents but the fact is that Sonny Liston, Muhamad Ali and George Foremanb were all dropped by lesser punchers who were in the same general size bracket. I think a strong case can be made that Jeffries had the best chion of all the lineal heavyweight champions. As for who might have KOd him you can obviousluy never say never but I would start with the elite heavyweight finishers such as Dempsey, Louis and Tyson.
Quite a few of the top class heavy champs would have stopped Jeffries,imo. Not many would do it with one punch,but it would by a wearing down process. Jim may have had a good jaw,but he'd have been wide open defensively.