3. Rocky Marciano vs. Jersey Joe Walcott
Marciano famously went 49 fights unbeaten before he finally retired. But Walcott was the man who came closest to beating him. In September 1952, Walcott was the champion and put a technical beatdown on Marciano for 12 rounds. He even knocked Marciano down in the first round.
But you could never count Marciano out. His right hand was a piston and he made it count in the 13th round. Walcott was sailing towards a decision win when he made a fatal error. He dropped his hands when Marciano let fly with a vicious right that smashed him into oblivion.