Philadelphia:
Obvious: Cheesteaks. If you see a place advertising "Philly" Cheesesteaks, then run. The sandwich is ubiquitous at Philly sandwich shops, pizzerias, bars, and even delis. But no place ever puts the word "Philly" in front of it. I also feel like I have to address some misconceptions. Onions are common, but green peppers and mushrooms are rarely requested, though most places have hot peppers available as a self serve option next to the ketchup and napkins. Wiz is the most popular cheese (bleh) and provolone and American are about equal. But the real reason it can't be made correctly outside of Philly is the bread. Don't even try to order this at Arbys or Subway.
Under the radar:
Roast pork with sharp provolone and broccoli rab. Many Philadelphians will argue that this is the better sandwich. It's a tough call.
Hoagies. May be known outside the region as a sub or torpedo? These are very common in Philly with the most popular being the standard Italian Hoagie. Ubiquitous at sandwich shops, delis, and pizzerias but if you don't have quality bread, the hoagie will suffer.
Irish Potatoes. Talk about a contradictory food. It's neither Irish nor a potato. It's a sweet bite sized treat of buttercream or cream cheese mixed with ground coconut and rolled in cinnamon; and historically made by Italian grandmothers and bakeries around St. Patrick's Day. Very sweet and highly addictive.
Philly Pretzels. Far better than the media-hyped crap they serve to tourists in NYC. Do native NYers even eat pretzels from the street vendors? Philadelphians OTH eat more soft pretzels per capita than anyone else.
Other regional specialties:
Trenton, NJ -
Tomato pie. Really, it's a pizza but with a very thin, well done crust and the cheese, used sparingly, sprinkled on the bottom. The sauce is the real star. Unfortunately, many of the original, old school tomato pie shops closed and/or followed their customers to the burbs. Are there any legit tomato pie shops left in Trenton proper?
Lancaster (Amish Country), PA -
Pretzels and
scrapple. Lancaster pretzels have been popularized by Annie Anne's but you can get much better. Buttery and delicious!
Norristown/Manayunk, PA -
Tomato pie. Different than the one in Trenton. Made in bakeries, not pizzerias. These are square sheets of thick, foccacia like crust with tomato sauce (ranging from smooth to chunky, sweeter to more savory) and a sprinkling of grated cheese on top and served room temperature. Very popular to serve these at birthday parties, football parties, etc.