Aguas Calientes, Peru
This is it, the most well known, visited touro desitnation possibly in all of South America. The lost city of the Incas, Machu Picchu, a pile of rocks, call it what you like. It really is something amazing, in the middle of nowhere, producing a magical feeling (sometimes hysterical) but also a real hassle to get to. I think that all this makes the experience so much more special.
Being such a world famous attraction, Machu Picchu is pricey. Just the entrance fee...