The Iguazú Falls are tremendous at any time, but their force depends on recent rainfall, so April (after the summer rains) is usually best. However, climate change is making the seasonal differences unpredictable here.
The Esteros are interesting year-round. The best time for seeing wildlife is spring and summer (September to April), though temperatures can reach an unbearable 45C in January. October and November are great, and so is March.