Would have to be Floyd vs Pacman in 2015 I reckon. I think that along with Floyd vs McGregor would have to be the 2 biggest audiences in TV viewing history.
Jake Paul vs Tyson was the most watched fight ever. That **** doesn't count. Certainly not the biggest fight. Maybe the most viewed.
Some reports say the rumble in the jungle had an estimated 1 billion world wide viewing audience. I have that as number 2 behind Floyd vs Pac.
Jeffries vs Johnson Dempsey vs Carpentier Louis vs Schmeling 2 Ali vs Foreman All of the above probably have an argument depending on criteria. Modern fights like Mayweather vs Pacquiao mostly just have the money earned as their claim to fame, but that's mostly due a greater ability to maximize profits off fights due to modern tech and business models.