I strongly suggest you to buy the pair of ray ban sunglasses which will be excellent at the design and perfect at the quality. You could just go to the real store and pick the one that you think will be suitable with your boy friend's face shape. The ray ban sunglasses will make men look so handsome and cool.
As you required, I think the definition of the best sunglasses can be considered from several aspects, such as brand, material, UV protection, and price. The best sunglasses have the feature of high quality, and sometimes they are more expensive compared with the ordinary sunglasses. Many reputed and high quality sunglasses brands such as Ray Ban, Dior, Armani, are all famous designer sunglasses, and they are the best sunglasses as for material and UV protections. As you want to buy a pair of cool sunglasses for your boy friend, I think the aviator with wraparound sunglasses would be the best choice. It has been popular for a long time, and it is also an essential part of every man's wardrobe. It is the symbol of fashion in the young men, which would make your boy friend cool and handsome.
It seems that your boyfriend is quite lucky. Anyway, from what I know, though there are lots of different makes of sunglasses, but some of them are especially made for men, such as Ray Ban, which just invented aviator sunglasses, and their aviator sunglasses are of great quality and supreme design. Thus, I recommend that you have a try.