The best dive mask is the one that fits your face.
A beautiful, high-tech mask is no use if it doesn't seal properly. Every face is different, so the first rule is to try it on before buying.
SEAL. The test is simple: place the mask on your face without the strap, inhale gently through your nose, hold your breath, look down and move your head from side to side. If it stays in place, it passes the test. A mustache or beard can affect the result.
COMFORT. Does it seal well? Now consider comfort. Silicone is generally the most comfortable material, though it costs a little more than siliprene or rubber. You'll wear the mask for quite a while underwater, so any discomfort will matter.
SIZE. Bigger isn't always better. Larger masks often offer a wider field of view, but their greater internal volume makes them harder to clear and equalize. Small or medium models are worth considering.
COLOR. It's a matter of taste. Clear silicone lets in more light and looks attractive, but can turn yellow over time and start to look discolored.