There are so many different varieties of cheeses. Each has it's own flavor and creaminess it brings to the table. But when it comes to a classic homemade stovetop mac and cheese you really never can go wrong with a sharp cheddar.
I also like to try and use at least one other type of cheese so I can get nice balance of both flavor, creaminess and cheesiness. I usually go with Colby Jack for my second option.