You may have been to Salto Ángel or Iguazú Falls, seen Niagara or not even be particularly interested in waterfalls, but it doesn't matter: go to Kaieteur Falls. Watching 30,000 gallons of water shooting over a 250m cliff (allegedly making this the world's highest single-drop falls) in the middle of an ancient jungle with few tourists in sight is a once-in-a-lifetime experience.
Depending on the season, the falls are from 76m (250ft) to 122m (400ft) wide. The trail approaching the falls is home to scarlet Guiana cock-of-the-rock birds, and miniscule golden frogs (best seen in the rainy season and/or in the morning) that produce a potentially fatal poison.