Date: 1948-56
Set By: Rocky Marciano

In the heavyweight boxing ranks, knockouts happen far more frequently than in the lower weight classes. One false move from someone like Marciano would send him to the canvas before he even knew what happened. So for a champ to not only go 49-0, but to also record 43 knockouts in the process, is simply not seen anymore. Even the Klitschko brothers have a couple losses on their records and several wins not by knockout. And don't talk to us about Floyd Mayweather, who fights in a lower weight class and purposely avoids both giving and receiving big hits in the ring (as proven by his winning just 26 of his 43 fights by knockout).