This is not waterfall-specific, but I am pretty sure that the Grand Canyon is not even close to the most-visited US National Park. Unless things have changed, that would be the Great Smoky Mountains NP, which straddles the TN/NC border and is about 45 minutes from my house. It has more than twice as many visitors as any other USNP. It doesn’t have the spectacular waterfalls or mountains as several of the west coast and rocky mountain parks, but the Southern Appalachians have their own quiet, mature beauty. The biodiversity is world class.