The hamburger plain and simple. No one else does it other than to emulate the United States. It is ubiquitous in almost every restaurant from top end to the local bar dive and usually gets top billing and generally a different listing on the menu from the other sandwiches. Fast food lives by the hamburger and reputations have been built - White Castle, McDonalds, Burger King, 5 Guys, In and Out, Culver's...you name it.
Chicken? I trust it in the Midwest and South fried but it is a specialty not generally available everywhere. Besides, other countries might do chicken better and in more ways other than fried in restaurants. Go to Bolivia, one of the poorest countries in the Western Hemisphere, for an eye opener on that.
Pizza? We do a damn good job with a rich variety but it is a specialty.
The hot dog? American, but again limited.
The deep-fried breaded pork tenderloin sandwich? God's gift to mankind and cardiologist but only appreciated in the Midwest.
In general, the sandwich, despite the Earl's name on it is pretty much an American institution.