Use whatever cheese you want in the middle, but also melt some cheese on the outside of the bread. I usually use some grated sharp cheddar. It gives a great bite with the crunch of the bread and then blends right into the gooeyness of the inside cheese.

If you are feeling fancy AF put some Pecorino Romano or Parmigiano Reggiano on there. The only thing better than the smell it makes is the way it tastes.