This has been discussed so many times here and elsewhere - please use the search function.
My personal opinion is, that you need sunglasses in the "wrap around" style, i.e. that they cover your eyes also from the side. Never use polarized, lenses which changing gradient or adjust their brightness.
I always prefered Oakleys, because they where the first to have optical abberations under control in curved lenses (especially important with corrected lenses), but Serengeti and others also seem to have caught up.
Always be aware for what you are using your lenses, and check the optical properties of your favorite: dark black for strong sun (tropics, deserts), yellow for haze or light sun. Oakleys always come with an extensive test protocol, so you can check how many percent of the light gets filtered. Don't forget to check the blue part of the sun light (that is the frequencies just above 400 micro). These tints are brownish.
Dani