What if Marciano had completed his plan to come out of retirement and challenge Johansson while Ingo held the title? Would there have been enough left of the old Rocky to beat him?
Marciano said he trained in secret for Ingo ,but couldn't knuckle down to the discipline necessary to round into the fantastic condition he was always in when he entered the ring,so dissatisfied with the results ,he aborted his comeback. Given this knowledge I must pick the Swede. Although Rocky in a BI interview clearly didn't think much of Ingo, I think he knew his own time had come and gone.
marcaino would start brightly, but get battered to bits by Ingmar later once he gassed. memories of Louis vs Marciano would flood back. also the problem of getting past liston if he needed to qualify for the fight, who would both scare him and batter him shytless.