I'm not really impressed by waterfalls. I rate them alongside sunsets as photographic subjects that are nice - but usually boring. I was born near Niagara Falls and have seen a lot of water falling over cliffs including Victoria Falls in Zimbabwe. I must admit I wasn't expecting much of Iguazu Falls when I stopped over on route from Buenos Aires to Rio. But oh how wrong I was!
The Falls lie on the border between Argentina and Brazil inside a world heritage protected national park. For a visitor the choice is to stay on either the Argentine or Brazilian side but my recommendation is to stay and explore both sides. In Argentina you walk on suspended walkways with tons of water tumbling beneath your feet while on the Brazilian side the view pulls back to sweeping panoramas enhanced by atmospheric mist and rainbows. You may recall that Iguazu was the setting for the 1986 film 'The Mission' and I half expected to see a Jesuit priest tied to a cross tumbling over the falls at anytime.
My advice is to stay at the two hotels right inside the park to take advantage of early morning and late afternoon light as well as to enjoy the peace and tranquility of having the place to yourself before busloads of tourists arrive later in the day.