Overall, the best time to visit China is either in the Spring or early Fall. My choice would be early Fall. Pandas are usually born June-August, so you'll have the best chance to see some babies. The weather in the southern provinces won't be too humid and hot.
In Longsheng, farmers will be harvesting the rice at that time, so in addition to lush hillsides, you'll also see a plethora of local people and water buffalo. I landed in Longsheng right after the harvest just as farmers started to ready the terraces for winter. The villages of Ping'an and Longi have a long history of farming this land, some families have worked here for hundreds of years. Ping'an village is the main touristed site with a catacomb of winding stairways and small shops. I made my way through the village streets and settled into the Ping'an Hotel, where my balcony overlooked the village below. This landscape is stunning and one of my most favorite in all of China. A short walk up Seven Stars Viewpoint allows you to see the terraces spread out beneath you like a scene out of a fictional tale. By the end of my hiking among the terraces, my heart swelled and my eyes could take in no more beauty. Be sure to explore the neighboring village (and less busy) of Longi. End the night with dinner at Ping'an and be sure to order the local dish of small fish.