Most famous waterfall in the world: Niagara Falls
This is a little bit subjective, of course. But Niagara Falls is considered the most well-known waterfall anywhere in the world. And the number of visitors it receives proves it. Niagara Falls gets somewhere between 9 and 28 million visitors annually (sources vary and really, no one actually knows). Even on the lower end, it's way more visitors than other waterfalls receive anywhere in the world. It has been the site of countless stunts, and more than two dozen films have featured the falls.
The flow has been reduced by half by power plants, and it is built up on both sides by businesses wanting to cash in on the tourist trade, but Niagara Falls is still a spectacular sight to behold -- thanks in no small part to over 100 years of pushback against commercialism.