The most commonly used cheese is mozzarella cheese. The BEST cheese to use is low moisture, WHOLE milk cheese. Experiment with different brands as some melt nicer than others. Most markets carry the part skim mozzarella cheese which burns faster and just doesn't melt as well.  A few well know pizzerias are using a combination of grated cheese, fresh mozzarella, and low moisture mozzarella, which is chefs kiss good!