If a hamburger is a sandwich, my favorite would be the Ultimate Cheeseburger from Jack-In-The-Box.  Cheese, meat, cheese, cheese, meat, cheese.

Discounting hamburgers, my favorite sandwich would be a fried egg sandwich.  Slice of white bread, mayo, a fried egg (with the yolk still runny!), a little salt and pepper, and another slice of white bread with mayo.  Fantastic!