A good chain restaurant, in my mind, doesn't necessarily serve the best version of a given food, in a given city. (Though it might!) What it should do is serve a reasonably tasty version of said food, reliably and at a reasonable price. It might not give you an A+ sandwich, but it should give you a B+ sandwich, at all its locations and all the time.
In London, there are quite a number of chains that I wish would make their way stateside; what chains do you wish would find their way to where you live?