The key to making the ultimate smooth creamy Mac & Cheese is using Sodium Citrate. This is the ingredient that makes American cheese so smooth and creamy. My recipe for the best Mac & Cheese is as follows. Use can mix and match the cheeses as desired keeping the same total weight of cheese.
210g pasta (I like using Medium shells)
270g Half & Half
8.8g Sodium citrate
110g Medium or sharp cheddar (Like Tillamook)
80g Gruyere
40g Swiss
Method
Cook the pasta el dente
Drain the pasta in a colander
Return empty saucepan back to stove
Turn on heat to medium low
Add the half & half
Add the sodium citrate & mix well
Slowly add the cheese
Mix until all cheese is melted (don't let it get too hot !!)
Fold the cooked pasta back into the cheese mixture
SmokinJim has it nailed...except I dont use the Greyere and Swiss. Sodium citrate is how Kraft Cheese made its name. You dont even need milk or creme. I discovered this when low carbing because making a bechamel or roux with flour and even using milk is NOT keto (cream is though but behaves different than milk). I used special low carb pasta back then, it was almost like regular pasta but super expensive (not that konjac crap!).
Even after going back to eating carbs more liberally, I still make my mac and cheese, and cheese sauce in general this way. The cheese sauce can be thinned a bit as needed with more water. It can be used for broccoli, cauliflower or even as a queso dip or poured over tortilla chips. I even make a pimento cheese version that is delicious.
I make the cheese sauce in the microwave. Much faster than in a pan.
Here's a couple recipes from the web. Most people dont do this because it requires the sodium citrate and for some reason think its bad for you. But, like MSG, I've done my reading on it and both are in my spice/seasoning collection all the time. Play around with it and make your own adustments/tweaks.
Pepper jack works fine. Whether you smoke it or not is up to you. This is a smoking site. People here will smoke a restaurant napkin or a flip flop...anything!

For me, everything doesn't have to be smoked, but in the summer, not using the oven inside is a good reason to cook outside and with molecular cheese sauce, you can just mix with the pasta and eat it just like boxed mixes. Baked mac and cheese is great, but stove-top is also very tasty.
This baked macaroni and cheese recipe harnesses the amazing emulsifying powers of sodium citrate to make the gooiest and silkiest cheese sauce. It's like the boxed stuff, but even better because we use cheddar and Gruyère.
www.seriouseats.com
One of the traditional dishes we took on in Modernist Cuisine at Home is classic mac and cheese. While we have always loved this family staple, there is an inherent problem with traditional preparations: all of the virtues of using good cheese are lost when you make a cheese sauce with flour and...
modernistcuisine.com