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!