You can really use any good melting cheese for this recipe, but I opt for either mild or sharp cheddar cheese and Monterey Jack. The cheddar brings the flavor, and the Monterey jack melts like a dream. Gruyere works really well in place of the cheddar. Sometimes, I use extra sharp cheddar and aged white cheddar cheese instead of the Monterey jack, but the result is not as creamy smooth. You can also substitute Swiss cheese for the Monterey Jack. Sometimes I sprinkle some grated Parmesan cheese over the top before it goes in the oven. I don’t recommend using pre-shredded cheese for this recipe. It often has additives that make the cheese more shelf-stable (like anti-clumping agents), which can cause issues in how it melts into the sauce. I've tried it with bagged shredded cheese and the sauce is not as creamy and smooth.