These falls though!! I'm torn between the Argentinian side and the Brazilian side, so I'm going to tell you to see both if you can. (The border crossing is pretty simple.) Although Argentina has more of the falls, and although it gets you closer to falls, the views from the Brazilian side are spectacular. Even if Iguazú/Iguassu/Iguaçu wasn't larger than Niagara, it would still be among the most memorable of experiences.
IGUAZU The falls of Iguazu are falls of the Iguazu River on the border of Argentine and Brazil. The falls divide the river into the upper and lower Iguazu. Approximately half of the river flows falls into a long and narrow chasm called “Garganta del Diablo” - The Devils Throat.
Yes, it's the gateway city to some of the world's most spectacular waterfalls, but the area in itself is also worth spending time exploring.
MUST SEE! Whether you come from the Brazilian side or the Argentine...do it! And take time to visit the other side while you're at it. Personally, I like coming in from the Argentine side and doing a day trip over to Brazilian territory. But I've done it the other way around too. Which is better, you ask? You say potato... If you find yourself in Rio or Sao Paulo, Buenos Aires or Salvador de Bahia, you should find a few days to go.