Why getting the size right is important with full face snorkel masks
The best full face snorkel mask is the one that fits you properly! Even the best quality, highest priced snorkel mask isn't any good if it's the wrong size. A mask that doesn't fit your face properly will feel uncomfortable. Masks that are too big will also let water leak in.
Always check the size chart of each brand as there are slight variations in sizing between manufacturers. Also make sure the supplier of the brand you intend to buy has developed a good customer service reputation as well as a good Returns Policy.
Regardless of which snorkel mask you look at, while there are hundreds of positive reviews on Amazon, there's also a few complaints and 1-star reviews on all full face among them about leaking. The main reason most full face snorkel masks leak is because they are not the correct size for the person wearing them.