It depends upon what you are trying to say.
"My favorite food is hamburger" implies that you like hamburger meat, which is used in many different applications/recipes. (For example, hamburger meat is used to make meat loaf, Salisbury steak, Sloppy Joes, and other dishes.)
"My favorite food is hamburgers" means that the meal you love best is a hamburger patty served on a bun with various toppings (like the type you'd buy at McDonald's).
- For Teachers