Hmmm I'd guess Louis- Schmeling 2 or Fight of the century. At its time Jeffries v Johnson was huge with the aftermath that followed. Numbers is not everything, hence why the aftermath of May-Pac was disappointing. Even the build up showed it was 2 men fighting 6 years too late.
In all seriousness you'd probably have to include: 1. Tyson - Holyfield 2. Crawford - Canelo 3. Mayweather - Pac 4. Fury - Usyk 5. Wilder - Fury II-III
Ali - Foreman Ali - Frazier Sugar Ray Robinson - Jake Lamotta Joe Louis - Schmeling Marciano - Joe Louis
Thrilla in Manila had 100 million PPVs according to this. https://en.wikipedia.org/wiki/Pay-per-view#List_of_pay-per-view_bouts
Hagler vs Hearns was big for its time. Fury vs Wilder 2 was a big one and same could be said for #3. The anticipation for Ward vs Gatti 2 was very high. And also of course Mayweather vs Pac Man and Ali vs Frazier
Not many alive who witnessed it and so far back most people have forgotten about it but Good v Evil ! With the whole World watching on the brink of World War 2 and the fight with the most political ramifications in history,the 1938 rematch between Joe Louis v Max Schmeling. Louis winning the fight for American democracy over Germany and its Nazi ideaology. Bert Sugar went on to say it was the most significant sporting event of the century .