Scrap mentioned Louis vs Schmeling, which probably is the biggest fight of all time, although I think Dempsey vs Carpentier had a buzz at least of equal to the Pacquiao-Mayweather matchup.

In the post-war era, I'd say the biggest were Ali-Frazier I, Ali-Foreman & Leonard-Hagler, although Duran-Leonard II, Tyson-Holyfield II & Oscar-Floyd were hugely hyped as well, even if the fights did disappoint.