I think you have the saying back to front. Berbick Could beat Ali at 45. George said, if you can't beat a man. For example, it is highly unlikely that Michael Moorer who couldnt beat him at 45 could beat George when George was 25. On to the original question, everyone seems to be ignoring the last part of the question which is that the reversal is when the older fighter is at a similar stage. So, Joe Louis clearly has the toughest job because the he fought Marciano in his last fight. In marcianos last fight he had maybe deteriorated a little but realistically he was not far off prime. Prime Louis vs Old Marciano is a pickem fight and could go either way because old marciano was still very good. Prime Tyson meant old Holmes, coming off 2 losses to spinks and in his first fight out of retirement without much training. I am not sure which version of tyson this equates to, but given that Holmes managed to produce good form against wolrd class oponents later on, we are probably looking at the Tyson that met McNeely (although you could also probably argue that the tyson that met lewis fits this bill also. Prime Holmes would definitely be capable of winning this fight, but then again Tyson could also catch and KO Larry like he did in the real fight if Larry wasnt careful. Holmes would probably start favourite, but tyson would be a realistic chance still and would have plenty of fans thinking he would win. Not all that far off a 50/50 fight if you ask me. Ali v Holmes. Holmes is always a bad style match up. The Holmes who had his last fight was still skilled, but not really a top 10 world class fighter of the time. Ali, imo, when in his prime peppers, toys with and stops Old Holmes the same way he outclassed old Archie Moore. Incidentally, prime for Prime, i also give ali the best chance at winning though the three fights could go either way. A real life rematch might be an interesting rematch. Ali v Holmes II would have went the same way as I despite the improvements vs Berbick, i think. An old Joe Louis did have a chance against Marciano, but it would be unlikely. Old Larry Holmes has the best chance. A fully trained Holmes that went the distance with Holyfield and shortly after might very well have beaten Iron Mike. When Holmes beat Mercer, again he would not be far off Mike. If Holmes Tyson II ever happens now, there is no reason why Holmes could not win given Tyson's total deterioration.
Ali would have won a good unanimous decision over Holmes, he would have taken it to another level in the championship rounds. Joe would've stopped Rocky late, around 13 rounds. Larry would stop Mike late, after going through some hell early on.
Holmes would have been especially dangerous for even a peak Ali...he was qite inlike anybody who Al ever fought. Ali wouldn't be puncher enough to ko Holmes, or even deck him IMO...and Holmes hit hard enough...along with his hand speed, to really bother Ali...not to deck him...but to hurt him. Marciano would have been down...and if it were the Louis of the Baer fight...it might have ended in a ko for the Brown Bomber. Holmes, if he had been as focused as he was for the Norton fight..without the arm injury that he when fighting Kenny, would have been a decision winner over Tyson. I obviously rate the bitter, nasty old peanut head pretty highly in both cases with Ali and Tyson.
I pick all 3 to be reversed. None are results I take particularly seriously when weighing up how good each man was.