OP - The traditional answer is "Cod", though my understanding is Cod has been overfish'd, heavily regulated now, and therefore rare~er and more expensive than it used to be.
In light of the reality here in the USA being "Whatever The Flakey White Meat Inside The Batter Happens To Be", I don't think a lot of us could say for sure.
And why have we (yet again) decended into a Fries versus Chips argument!?!?
>>>> And for the record <<<<<
In this part of the USA:
(1) They're only "Steak Fries" when they're cut as large as (or larger than) a man's thumb. Not a whole lot of places do that any more.
(2) Hash Browns are made in a frying pan or griddle (stovetop), unless you eat at McDonalds
(3) Roasted Potatos are done in the oven - preferably serving as a bed for a whole chicken. And preferably both lavished with fresh rosemary