Which one is correct?
My favourite food is hamburger.
My favourite food is hamburgers.
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).