The Victoria Falls

Visiting the falls depends on whether you want to view the Falls at its most impressive or at its lowest flow. For example if you want to view the falls up close you will want to be there at its lowest flow.
As there are numerous factors that come into play it is very difficult to predict the best time for doing both.
The Falls is in its highest flow around May-June time frame. You could then expect the falls to span the entire 1.7km width of the Zambezi River as it plunges over its 108m wall. Unfortunately, during this time most of the viewpoints are too wet to photograph effectively (careful not to destroy your equipment by exposing it in the rain) and Raincoats are a must. This would be the perfect time to view the Falls from the air.

The Falls is in its lowest flow around October-December time frame. During these conditions the falls will brake up into subgroups comprising of seven smaller water “strands”, with much of the underlying bedrock exposed along the wall. Lacking its thunderous impact due to some degree of water diversion for hydroelectricity, but plenty of other activities become possible. Among these activities are swimming at the “Devil's Swimming Pool” that lets you swim right at the edge of the Falls!
And other water activities in the Zambezi River below the falls becomes possible. Mist obscuring your viewpoints won't be as much of a problem.

During July through September is perhaps the most popular time frame to visit the Falls (good views and best weather). Gradually in transition from a flooded state to a low flow state with an acceptable volume of water and fewer problems with mist.
This would probably be the best time to go in terms of getting the best of both worlds taking in the impressive visual impact of the Falls and partake in the most activities.

Around January through March/April is the wet season. The area is Warm and extremely humid and thundershowers are more likely. During this period the Falls are in transition, going from a low flow state to a high flow state. Thus, you're likely to have satisfying views of the falls without mist being as much of a problem as during full flood. Even though it is typically raining this time of year, the flow of the River depends on the drainage systems further upstream where water drains into the Zambezi River from Western Zambia and Angola.

The Zambezi basin above the falls experiences a rainy season from late November to early April, and a dry season the rest of the year. The river's annual flood season is February to May with a peak in April, The spray from the falls typically rises to a height of over 400 metres (1,300 ft), and sometimes even twice as high, and is visible from up to 50 km (30 miles) away. At full moon, a "moonbow" can be seen in the spray instead of the usual daylight rainbow. During the flood season, however, it is impossible to see the foot of the falls and most of its face, and the walks along the cliff opposite it are in a constant shower and shrouded in mist. Close to the edge of the cliff, spray shoots upward like inverted rain, especially at Zambia's Knife-Edge Bridge. As the dry season takes effect, the islets on the crest become wider and more numerous, and in September to January up to half of the rocky face of the falls may become dry and the bottom of the First Gorge can be seen along most of its length. At this time it becomes possible (though not necessarily safe) to walk across some stretches of the river at the crest. It is also possible to walk to the bottom of the First Gorge at the Zimbabwean side. The minimum flow, which occurs in November, is around a tenth of the April figure; this variation in flow is greater than that of other major falls, and causes Victoria Falls' annual average flow rate to be lower than might be expected based on the maximum flow.