So many great recipes are available, as thenamesdave stated. The only constants are that the mac & cheese should be creamy smooth in texture and have great flavor. The smoothness comes from making a great cheese sauce, either as the mac & cheese casserole is baking, or in a pan, and then adding it to the cooked pasta. The casserole should have a light crunch on top, from baked cheese, but not burnt.
Typically, sharp cheddar adds flavor, while a great melting cheese helps to make it creamy.
I prefer to make my sauce separate, and add it to the cooked pasta. This allows me to better control the moisture content (who wants soupy mac & cheese, or mac & Cheese that sucks the moisture right out of your mouth!). I like to use a combination of cheeses such as sharp, white cheddar that has aged in excess of 3 years, maybe some grated colby, and cheeses like Fontina, or Butterkase, or Bergenost (I've had it from yancy's Fancy only once, but fell instantly in love with it), or Velveeta (I know, it glows in the dark

), all combined with whole milk, or half 'n half. Add a touch of good mustard, a a little freshly ground black pepper and you have a great sauce in the making. For stronger flavor, use more cheddar. For a more mellow flavor, decrease the cheddar and use the other cheeses, or add Monterrey Jack, or Muenster to the mix.
Finely grate all cheeses and allow to come to room temperature. Heat a cup or two of milk in a medium sauce pan until it just starts to simmer. Remove from the heat. Slowly add the cheese while stirring, making sure it is completely melted into the sauce. Keep adding cheese until your get the desired thickness. Add the pepper and mustard to taste. Combine with the cooked pasta in a casserole dish, sprinkle on some more cheese, and bake at 350 for about 30 minutes.
Serve this with some good, braised chuck roast, or Pork Sirloin roast, and a green veggie, I'm thinking steamed asparagus as it will perfectly compliment the meat and mac & cheese. I'm now wishing I had that plate sitting in front of me right now. Oh yehhhhh.

Seeeeeeya; Goodweed of the North