You need these cheeses to set a melty, delicious precedent for your mac. These can be used alone, or as a necessary base for a combination of cheeses.
"Great first option, it's a classic. You can't really go wrong and you get the freedom to play around with different levels of sharpness -- and it makes an excellent base to be paired with other funkier cheeses. This is the standard." -- Ciardi
"Great meltability factor -- it's a grown-up cheese. This cheese is easy to work with, and will really get that 'grown-up mac & cheese' feel down." -- Jones
"Also an excellent alternative to cheddar as a base -- it's a great semi-firm cheese. I would recommend adding your own peppers, instead of opting for something like a pepper Jack, though. It's not that much more difficult, and makes it way, way better." -- Ciardi