I'd have to respectfully disagree on Tyson vs Foreman Tann. Foreman was a force of nature and was outsmarted and psyched out by Ali IMO. Tyson is not in the ballpark of being as smart as Ali. More importantly, prime Tyson won many fights through sheer fear. Foreman would not have been afraid of Tyson and my verdict would be a Big George KO victory in the mid rounds.
I'd have like to have seen prime Liston vs prime Tyson without mob interference. Tough to call as they were similar but I'd go with Liston in a fight that could go either way. It would end in a chilling KO either way though!
Other fantasy matches from other weight classes: (Sorry couldn't resist)
Roberto Duran vs Floyd Mayweather @ lightweight
Duran via late KO. Mayweather is a brilliant fighter but would never have coped with Duran's ferocity. Duran is probably the greatest ever 135er.
Carlos Monzon vs Marvin Hagler @ 160 Middleweight
Monzon via close decision? Actually very hard to call. Two all time greats in battle. I can't separate them since both fighters had very few weaknesses in their respective primes and went on long unbeaten runs after bad early starts.
No excuses, just results.
Getting it done one session at a time.