The fight between Muhammad Ali and George Foreman is claimed to have a billion viewership.
For example Jet (Sep 1995):
the first one-billion viewer fight
reportedly reached one billion viewers,
This is quite hard to believe since the world had four billion total population at the time so one in four people would've needed to watch this. I am skeptical there were even enough televisions to do this. For example, Chinese Television and Radio claims "In 1978, there were 1 million television sets in China". India Media Stats claims "The number of Indian households with a TV set went from less than 1% in 1975...".