La Mezzetta, open since 1939, is well known not just with the locals but with foreign visitors thanks to Somebody Feeds Phil on Netflix.
La Mezzetta keeps things simple with no sit down dining and only four options: muzzarella, fugazzetta, anchoa y napolitana.
You'll queue up on the sidewalk and order and pay from the window outside. Take your receipt and go inside to wait for your slice and eat standing at the counter (or order a full pizza to take home).
What to get? Don't think twice and get the fugazzetta (the cheesiest version of this already cheesy flavor I have ever seen).