The best cheeses are ones that melt but do not retain their shape when melted. Semi-soft cheeses like Gouda, Edam, Stilton, or Mozzarella or firm cheeses like Cheddar, Parmesan, Gruyere, or Emmental. Avoid dry or fresh cheeses like feta, queso fresco, Halloumi, or fresh mozzarella. These will either become grainy or won't melt into a sauce. These are great options for toppings though, used fresh or warmed to start to melt.