You want to use one or more cheeses with great melting properties. Try Colby, fontina, gruyere, or muenster. You can also add cheese for flavor – gouda, feta, romano, and blue-veined cheese will add zing and punch to your slow cooker mac. For these less melty cheeses, just be sure to shred them well. A cheese like sharp white cheddar may be the best cheese for mac and cheese because of its intense flavor and great melting properties.