I actually think it has to be Ali / Frasier for the significance of it. But the Barrera / Morales one was also a "true rivalry" with real bad blood.
What about Duran-Leonard or Leonard-Hearns? Greb-Tunney? Wills-Langford? Pep-Saddler? There are so many grat rivalries but i think Ali-Frazier was the best - and the most hyped.
I loved Barrera Morales, and its basically the one I am most familiar with however Ali Frazier is probably the most popular
that is an excellent selection! I would have said Mora vs Manfredo i mean he even named his ***** after Peter out of spite but nobody knows all of that so I chose Ali Frazier:hat