Custom-fitted: These are made specifically for you by your dentist. They may be more expensive than the other versions, but because they are customized, usually offer the best fit.
Stock: These are inexpensive and come pre-formed, ready to wear. However, they often don't fit well. They can be bulky and can make breathing and talking difficult.
Boil and bite: These mouth protectors can be bought at many sporting goods stores and drugstores and may offer a better fit than stock mouth protectors. They are first softened in water (boiled), then inserted and allowed to adapt to the shape of your mouth.
The best mouthguard is one that has been custom made for your mouth by your dentist. However, if you can't afford a custom-fitted mouthguard, you should still wear a stock mouthguard or a boil-and-bite mouthguard from the drugstore.