Louis vs. Schmeling II Not just the biggest fight ever but the biggest and most significant single sporting event in history. There is no close second with any criteria beyond the quantitative.
The most discussed, most watched, and highest generating was Mayweather vs. Pacquiao with no close second. I would say the answer is Louis/Schmeling II and Mayweather/Pacquiao. Between them both they cover all rational criteria for determining "the biggest fight ever."
Louis vs. Schmeling 2 followed by the first Ali vs. Frazier fight in 1971. The exhibition last May perhaps was the highest grossing revenue fight. It wasn't anything near the cultural significance of the two fights listed above.
You didn't see the photos of people gathered around televisions all across third-world African and Asian countries? You didn't pay attention to the 5 year buildup to the fight? You didn't notice that just about everyone in the world watched it?
I'd say Dempsey vs Willard or Louis vs Shmeling. Both were when boxing was as big as baseball and basically the entire country was invested in the fights and listening to it on the radio. Didn't Dempsey vs Willard have over 100,000 in attendance? And Louis-Shmeling had the political thing going, as well as it was the HW championship of the world. Mayweather -pacquiao PPV wise sure, but in this age where boxing is a niche sport, I don't think it compares to those two
Lots of people watched it. As stated in the reference to probably being the highest revenue fight. Doesn't make it the biggest or most important ever. The little Heavyweight title fight in 1938 may have had a little more cultural relevance.