I"m old school, so I have to go with Ali-Frazier. ALi-Frazier 1 was the only time in boxing history that 2 undefeated and prime HW champs clashed, although Ali was sort of rusty, the fight lived up to the hype and more, plus this is the glory division of boxing.

Bowe-Holyfield trilogy would be a close 2nd.