Mac and cheese or any casserole I like to use Gruyere and sharp white cheddar in white sauce. I'm a little old fashioned.
For cheeseburgers and toasted cheese sandwiches a thin slice of Limburger along with a slice of medium sharp cheddar or year old longhorn. I use what I can get but those are my favorites. Sometimes a thin slice of ham or crispy bacon on the toasted cheese.
A thick slice of Bermuda or Vidalia onion completes the sandwich.