What is the best way to reheat a sub-style sandwich?
I would suggest opening the sandwich so the filling is facing up, removing any toppings that you would prefer to remain cold, and sticking it in an oven or toaster oven for a bit. That gets the heat to the center of the sandwich immediately, though you'll want low heat (maybe 250F?) and not to heat it very long or the bread will get too toasted. It won't ever be as good as the day before, but I find that microwave reheating tends to result in poor-quality bread.