My sandwich choices are pretty limited to begin with, but I really love any kind of veggie sandwich, honestly. I usually make mine with lettuce, spinach, tomatoes, cucumbers, onions, some good deli cheese, and some dijon mustard and a light spread of olive oil mayo. Good stuff.
Although I sometimes put fried eggs in my sandwiches, that's pretty good too; or boiled eggs cut into slices.
(my heart goes weak for a good chunky peanut butter and jelly sandwich though.)