Sun Heung Yuen
Time Out says
Founded in 1968, Sun Heung Yuen (Kin Kee) is famous for its egg and beef sandwich which attracts hordes of people to come. The beef is marinated and then fried with eggs, so the sandwich is full of fillings. You can add $1 for toast the bread. The eatery is open 24 hours, and there is usually quite a few people eating late at night.