Pret's sandwiches hold up to any sandwiches I've had in any of those countries. I've had the occasional better sandwich (especially in Paris), but I would take Pret's sandwiches over most of the crap sandwiches I ate in New York. New York sandwiches tend to assume that packing in TONS of shit quality meat makes the sandwich better. I don't actual like to eat through an inch of American-quality processed meat to mask the fact that the bread is crap.
French style sandwiches have less total filling but the filling itself is high quality and tasty, including the butter. Also the bread tends to be extremely delicious by itself.
The only sandwiches I would get in New York are freshly made bagels from the better bagel places in Brooklyn. However, this is comparing apples to oranges because this whole discussion is about pre-made sandwiches which you get off a shelf and go, not freshly made sandwiches.