Strong emmenthal Swiss cheese is the best. None of that bullshit Boar's Head baby Swiss; it needs to be imported. It's the perfect cheese--you can eat on its own and it melts the perfect amount.
There's also this french cheese called Comté, that's made in caves in France. Each batch is different, but it has this really delicious nutty quality to it.
For soft cheeses, I like Brie de mieux (which is unpasteurized and therefore illegal in the states; sorry burgers); more interesting than the pasteurized stuff if you're just going to eat it.
For hard cheese, pecorino romano or parmesean reggiano with some cracked black pepper and a drizzle of honey.