Mask, Fins, and Snorkels are the BASIC equipment requirements for divers.  Above you see 3 different styles of dive masks available.  In reality there are a lot more. Leisure Pro list 27 different manufactures of dive masks, 6 different window styles and another options, such as color and prescription lens.  Like many divers, I have 3 different masks.  I have a rose colored prescription mask for deeper diving(it keeps the colors more visible), I have a low volume mask with 2 lens, and a low volume single lens mask. 

Buying a mask has 2 really important requirements.  Make sure the mask fits properly and is comfortable.  It is pretty easy to make sure the mask fits.  Place the DRY mask over you nose and eyes (just like you are going to wear it) without using the strap, and make sure it is pressed against your face.  Inhale through your nose and see if the mask will stay on your face by itself, without holding it.  If it falls off it is probably not sealing well enough to fit your face.  Next pull the strap over your head and test the comfortability.  The comforability is your perference.

Different sources have rated masks such as The Oregon Coast Aquarium, Scuba Diving Magazine, Nextag and more.  Which ever mask you buy, make sure it seals or fits, and make sure it is comfortable.