Pizza Shack is a Pittsburgh institution. It's not in the heart of the city, but off 51 in the South Hills. It's easy to pass by without a second glance, especially because it is closed most of the time.
Call ahead for a pizza, and it's guaranteed that (if they're open) they're very busy at any time of night. I recommend ordering a plain pizza, but all of the toppings I have tried are pretty good too. Plain just hits the perfect balance of flavour, in my opinion. When you come to pick it up, sometimes you can grab a chat with the owners, who are friendly and quirky to talk to.
The pizza comes as a square. Some of the pieces have crust, others do not. The crust it crispy and almost like a dense pastry, but the sauce is sweet like stewed tomatoes and jam... or something like that. Just try it. It's so worth it.