Heat oven to 350F degrees. Grease a large glass casserole dish with butter (8.5x11 or larger). Set aside.
In a large stock pot, cook pasta of choice until just al dente. Drain and set aside (Don't worry if the pasta starts to stick together. We'll take care of this during assembly).
While pasta is cooking, pour milk into a large glass measure or other microwaveable container. Heat until warm, about 2 minutes on high.
While milk is heating in microwave, melt butter in a large skillet over medium heat. Once butter is melted and just about to bubble, add flour. Stir, heating till just golden, about 30 seconds. Turn heat down to medium-low.
Add 3 cups milk very slowly, stirring with a whisk to help smooth out any lumps. Once all milk is incorporated, turn heat back up to medium or medium-high and stir until sauce starts to thicken and bubble slightly, approximately 5 minutes.
Add salt, garlic powder and cayenne.
Adjust heat back down if necessary and add cheddar, mozzeralla, swiss, gouda and Parmesan cheese. Stir until all the cheese is melted. Taste and add more seasoning as needed. If cheese sauce seems too thick for your liking, add up to one more cup of milk, stirring to incorporate and maintain smooth texture.
Set up your casserole dish next to the cheese sauce and get the extra shredded cheddar cheese ready (1 to 2 cups). Pasta is probably stuck together now. Just splash the pasta very briefly with either cold water or olive oil and toss with your hands to separate.
Pour just enough cheese sauce into the bottom of your casserole dish to cover. Add a layer of pasta to cover the bottom, not too much. You want the layer to be thin enough so the sauce can seep down. A bit more than a single layer. Sprinkle grated cheddar cheese in thin layer over pasta. Pour enough sauce over this layer to cover completely, almost half of the sauce in your pan.
Repeat with more pasta, remainder of cheese sauce and cheddar. Cover with aluminum foil.
Bake, covered for 30 minutes. Meanwhile, place crushed saltines in a gallon-size plastic bag with 2 tablespoons melted butter. Sprinkle with a dash of cayenne pepper and/or red pepper flakes. Add a dash of black pepper for even more of a bite and nice color.
After the casserole has baked for 30 minutes, remove foil and sprinkle the crumb mixture over the top, completely covering. Place back in the oven and bake uncovered until bubbly at the edges, approximately 15-20 minutes. Crust should start to turn golden.
Remove from oven and serve immediately.
Notes: *You can leave out the "heat" spices if you must, but if you can sneak in even ¼ teaspoon cayenne pepper into the sauce, you won't taste heat but it adds a nice zip of flavor.
*You can just use a whole bunch of cheddar cheese if you don't want to add all the varieties I list in the ingredients and it will still taste fabulous. The mozzarella is a nice stringy binder, swiss and Parmesan add a sharp but subtle bite and the smokey goodness for the gouda, though also subtle, really takes the flavor over the top.
*I've made this mac 'n cheese with cubes of ham or pancetta before but if you want to go crazy-fancy and give this dish even more of a Wow factor, add crab or lobster just before baking.