All depends on your interests and how to appreciate the scenery

There are all kinds of flowers on full blossom in May, red maple leaves in October. We have a lot of rain in June and also  heavy mist on the top then. But when it gets clear, the verdure of the bamboo groves and tea bushes all over the whole areas is a great attraction to the professional photographers, to whom June is worthy trip to Mt. Huangshan.

July and August might provide most of the clear days for the whole year,with blue sky and strong sunshine.

for detailed info, please consult http://www.shanghai-huangshan.net