If you asked me who was the better fighter, the answer is Marciano, hands down. Since your asking prime vs. prime, the answer would have to be Klitschko. The reality of the matter is, sports nutrition has improved a thousand-fold since the days of Marciano. Klitschko is from a more civilized era of boxing, and his strength and conditioning are superior to that of Marciano's. Now, if Marciano was born in this era, and had access to the nutrition and training techniques that heavyweights have today, then my wager would be that Marciano would mop the floor with Klitschko. But we're talking about two completely different eras of warriors here. Marciano had the heart and ability of a true champion, and with the proper training tools, yes, he would have won. But Marciano in "his" prime during "his" era would not have stood a chance against Klitschko in "his" prime and "his" era. Does that make sense?