Sunglasses that are best for beach volleyball should have 100% UV protection because beach volleyball is an outdoor activity and your eyes are exposed to UV rays directly. A pair of sunglasses with 100% UV protection can help protect your eyes from being damaged by the strong light and harmful UV rauys. What's more, sunglasses for beach volleyball should be comfortable to wear, which will allow you to focus on the ball easily.
I don't know what is the "best" sunglasses for beach volleyball. But i recommend you polarized sunglasses. Compared regular sunglasses, polarized sunglasses can prevent UV rays, block glares from water surface as well as reduce the strong light from sunshine. If you want to buy brands sunglasses, Oakley are recommended which is famous in carrying sports sunglasses.