Well, which variety are we talking about?
For a thin crust, I really like John's in downtown NYC. They only serve pies. Sorry, no slices. Everything is super fresh, and it's just fantastic.
For deepdish, Chicago really made me a believer. I recall a couple of places in Chicago that were chains that made a fantastic deep dish pizza. Giordano's was one. I'm spacing on the other, though. But when I was going to Wizard World Chicago about 3 years running, deep dish pizza was always on the itinerary.