I prefer to use a white flavourful cheese that has really good melting qualities. Swiss cheese is a great option, but my favourite cheese to use in a tuna melt is a melty cheddar cheese such as Balderson. Gruyere, Emmenthal, Fontina, Monterey Jack, Pepper Jack, or Colby, would all be great options. I would try not to use mozzarella becasue it has very little, if any, flavor.