@GaryPoole - for a national/regional chain hands down, 5 Guys.

But my definition of fast food is that it is delivered to in in less time than it takes you to eat it. Only Blackies Hot Dogs fits this and is actually good.

But having only 1 food on the menu and decades in the business allows you to have exactly the right amount of food ready at exactly the right times.

You walk in, they ask 'how many and what do you want to drink" before you even get to the counter.