Vitali Klitschko vs. Rocky Marciano.

Wlad is an extremely gifted boxer. To say otherwise would suggest that you have no first hand experience of the sport. He is however a sportsman and not a fighter, which accounts for a lot in boxing and although exceptional has more of a stylistic weakness than other great fighters. Which makes him vulnerable H2H against other all time greats with certain styles.

Rock would be completely overpowered in every way physically, he has heart and intangibles on his side and that's it.