Our favorite cheese is probably Jarlsberg, as inspired by the movie The Devil Wears Prada. It's a very mild cheese with a delightful amount of melt. If that's not available (but we usually have it on hand), we'll use Velveeta. Velveeta may be unnatural but there is something about the melty gooeyness of Velveeta "cheese product" that makes a lovely grilled cheese.
I'll admit to using whatever bread we have around - usually plain old sandwich bread - slathered in a lot of butter. Most often my cooking method is either a frying pan on the stove or our beloved Cuisinart Griddler. It's a much more squishy sandwich but still very yummy.
If we plan ahead, we're much more likely to experiment with other cheeses and breads. I haven't done much as far as adding things to the cheese other than occasionally some ham but I imagine tomatoes would be very yummy.
So, fix it! Tell me about your grilled cheese sandwich. Inspire our comfort food so we can be prepared for the falling leaves and frosty afternoons.