What to Look for in a Mask

Face and Ear Guards
Masks are designed to protect your face, so the first thing you should look for is well designed face and ear protection. All masks will protect you from serious injury, but more rigid masks will better protect you from smaller bruises, though they are also more likely to crack. Make sure that your mask adequately covers your ears and chin (if you have a long face, get a bigger mask). If you are very worried about getting hit on the head, there are some masks that will cover your entire head, similar to a motorcycle helmet.
Lens
When looking at the lens pay attention to the field of vision it offers and whether or not it is a thermal lens. The human field of vision is about 180 degrees and staring straight ahead it is even less. Unless you are planning on looking out of the corner of your eye regularly rather than turn your head, a mask with a 120 degree field of vision should be more than enough. Thermal lenses are simply two separate lenses with air in between them which helps prevent fogging. Some lenses are also tinted and function similar to sunglasses.
Padding
The foam padding inside masks may not seem like a big deal, but some good foam will make or break a mask. Look for padding that will mold to your face but also returns to its original shape. Look at how the foam is attached to the plastic mask - check for a good solid glue job with no corners unattached. And most importantly, make sure you like how the foam feels on your face - it will only break in after a long time, so uncomfortable foam to begin with will be uncomfortable months later.
Fit
Look for a mask that fits firmly on your head without pinching. You should be able to tighten the straps to make your mask snug without maxing out the adjusters or permanently stretching out the straps. Make sure that your mask comfortably fits around your ears and that the mouth guard doesn't hit your chest when you look down or your shoulders when you look to the sides.
Other Things
Most masks come with visors which I have found to be very useful when playing outdoors. Additionally, some fans come with built-in fans designed to reduce fogging. The plastic of masks comes in numerous colors ranging form bright reds and blues to dull blacks and greens, so you'll be able to find a color you like. There are also many purely cosmetic upgrades you can add to customize your mask.
Overall Value
Paintball masks can cost from less than $20 to more than $100. Figure out what features you really need (for example, some people never have problems with their lenses fogging, so they might not want to pay extra for a thermal lens) and then see what masks fit your budget. The best mask is not necessarily the most expensive; it's simply the mask that works best for you.