Much has already been written (and argued) about the right cheeses to use. We think it's better to try ourselves.
A classic, rather spicy mixture is comprised of a third each of Gruyère, Freiburger Vacherin and Appenzeller. If this seems too substantial or even too specific, then you could also use a simpler hard cheese, like Emmentaler, and combine it with a third to a half portion of Appenzeller.