I'd go with mushroom25 on that - It's got to be one of the best. However if we are talking solely about visitor numbers there are others like Rio, Acapulco and Buenos Aires. Also some that are gaining in popularity. The Galapagos Islands, the Venezuelan rain forest and the amazon delta are all becoming a big draw for 'eco-tourism' . There is also the 'inca trail', but that lost popularity recently due to the re-emergence of the 'shining path' bandits.
Iguacu Falls on the Brazilian-Argentine border would rate at the top of the natural attractions I would think. At 2.5 miles wide it plunges down 260feet to the river below. At peak flow it puts enough water over the falls to fill 6 Olympic pools EVERY second. Some show! Also, it is reasonably accessible.
I walked across the top of one of the Iguazu falls once - not the devil's throat one, obviously; one of the side ones - and found the water was about 3 inches deep and slow-moving. It doesn't take a lot of water to produce the appearance of a big waterfall. But I think the answer is probably Machu Picchu. Rio must get a lot of visitors but it isn't really an attraction in itself.