One of the most famous waterfalls in the world, Iguacu Falls is located between Brazil, Paraguay, and Argentina. To get a sense of its scope, they are twice as wide as the Niagara Falls and much taller. This network of hundreds of waterfalls is a truly breathtaking spectacle, and can be seen from numerous angles and degrees of immersion (i.e. boat tours, a number of viewing platforms that bring you right into the noise and mist), although you'll get more of a panorama of the majority of the falls if seen from Brazil.
The Argentinian side has a greater selection of walks and closer views, although the Brazilian side is especially significant for the Devil's Throat, which has the largest volume of water in the falls. Unless you plan on driving a full day from any large city in Brazil, it may be worth your time to take a flight into Foz do Iguacu, the city nearest the falls. There is reliable busing from there. Entrance to the falls is 41 R$ (about $22 US), while it costs 100 AR$ (about $23 US) to visit the Argentine side. To get the full experience, we suggest doing both!
Location:Foz do Iguacu