It may not be the biggest or the highest, or the most watery, but - for my money - it has to be the Kaieteur Falls (Guyana).
I flew up there once, on a day tour from Georgetown. After a while, you get to the upper reaches of the Potaro. This river itself would have been worth the trip, like a gorgeous seam of folded jet. But then suddenly it just ended. It poured over a lip, and vaporised as it fluttered away into a deep abyss. The Falls.
Without having seen them, they’d have been hard to imagine. Every minute, the equivalent of twenty-four Olympic-sized swimming pools was falling twenty-seven storeys. In other words, one moment a river the width of the Thames was looping through the jungle, and the next it was dropping from a ledge the height of Telecom Tower.
Somehow our little plane managed to land on this ledge, and we all got out and crept towards the rim. It was like watching all the whisky in the world tumbling into an explosion of weather. Way below, we could just make out the river, reassembling itself from the clouds, and then swirling off through a canyon. Conan Doyle would have loved it up here. It would have confirmed his belief that there were Lost Worlds, and that they were right here in Guyana. Amongst the oddities living on the edge were swifts that could fly through hundreds of tons of falling water, and a tiny yellow frog that spent all its life on a single bromeliad, swimming in its cistern.