I think the best cheese combination for a grilled cheese sandwich is to have one melty cheese (e.g., gouda, fontina, mozzarella) and one flavorful cheese (e.g., sharp cheddar, gruyere). This guarantees that you’ll get a stretchy, gooey filling which is also salty and tangy. My favorite combo is sharp cheddar and gouda. I’ve gone for one slice of each but if you want more stretch, add another slice of the melty cheese to your sandwich before toasting.