I use medium cheddar, monterrey jack (not pepper jack), a little smoked gouda and last time I added a little bit of my smoked sharp cheddar. Keep the sauce thin.
Honestly I was hoping to be lazy and pick up the Costco Mac and just mix some brisket into it but they were out by the time I got there last night so I went to HEB to get the raw ingredients I needed. For cheese I went with Boars Head smoked Gouda and extra sharp white cheddar.
My go to is to start with a traditional flour and butter based roux with some garlic and mustard powder added in. After trying many many many cheese combinations through the years the winning combo is gruyere, smoked cheddar, and yellow american. Can't be beat!