I call this the ultimate because of the sauce — it's perfectly smooth and creamy, just the way I like it. If you prefer a less smooth cheese or a different flavor profile, feel free to substitute your favorite cheeses in. Just remember what Laura Werlin says when it comes to cheese: an infinite number of cheeses can go into your mac and cheese, but what matters is the results you're looking for. Choose the cheeses that will give you those results.
Preheat the oven to 350 degrees.
Add the cooked pasta to a greased 9-by-13-inch glass baking dish.
In a medium-sized saucepan, melt the stick of butter over low heat. When the butter is melted, slowly whisk in the flour and whisk well until its evenly incorporated into the butter. Let the roux cook, while whisking, over low heat, until a buttery-baked smell permeates and the roux starts bubbling all over the surface. Slowly add the milk, whisking constantly, and then increase the heat to medium. Season the bechamel with salt and pepper to taste, and then add the nutmeg. Let the sauce cook over high heat until boiling, and then remove from heat and add the cheeses (except the Parmesan). Stir well so that they're all completely melted.
Pour the sauce over the pasta in the baking dish and, using tongs or a spoon, toss well so that each piece of pasta is coated well with the sauce. In a small bowl, melt the tablespoon of butter and add the breadcrumbs. Stir well, and then add the Parmesan cheese. Sprinkle the breacrumb mixture evenly on top. Bake the mac and cheese in the oven until bubbling and golden brown on top, about 30-45 minutes.