OK, my research has advanced to the point that I can forward two candidates. Now, when I say "sandwiches," I mean something you could eat everyday for lunch, and not "Sloth tongue with Antibean bee's jelly butter and a lacy crust of fried morel tarts" or anything like that -- some restaurant may make such a dish, and it may be fantastic, but you aren't going to be making it for your lunch. No, I mean sandwiches with readily available ingredients that cost a couple of dollars and that you can make in under 20 minutes. Also, I am looking at when the "sandwichness" itself is what is so good about them -- of course, if you take a fantastic piece of steak and place it between two decent slices of bread, it will taste good -- but not as a sandwich, but as a fantastic piece of steak that happens to be between some bread.
So, here are my two candidates (not ranked in order):
1) The Reuben
Ingredients: Pastrami (cut it out with the corned beef, already), swiss cheese, sauerkraut, Russian dressing, rye bread.
2) The Cheddar Ploughman
Ingredients: Cheddar cheese, cucumbers, lettuce, tomatoes, mayonnaise, Branston pickle, brown bread. (I've eaten lots of these, and I know the igredients can vary quite a bit -- this is just my favorite combination.)
What these sandwiches have in common is that their excellence stems from a blend of surprising ingredients that you might not think would go together at first, but that somehow merge into a single taste that seems greater than the sum of its parts.
A Note on the English Versus the American Sandwich:
The are two quite different sandwich making philosophies at work here, and like Aristotle's types of constitutions, they each have their good and their degenerate variety.
The English style is to thinly layer each ingredient in balance. When done well, the result is something that tastes more like a single food than any American sandwich does. When you encounter the degenerate variety, you have just paid £2.95 for two slices of bread.
The American style is too lay the ingredients on more thickly. When done well, it never tastes as unified as the English style can, but you get a nice hearty meal and a clear taste of your ingredients. In the degenerate version, encountered especially in rural areas where "value" means lots of calories per dollar, you are faced with eating a lump of a half pound of roast beef, upon which a few scraps of lettuce, tomato, and bread seem to have gotten stuck.