Required? I'd go with water, flour, yeast, salt, olive oil.

After that it becomes difficult as a pizza bianca with nothing else but olive oil would be strange, you generally get at least garlic and some cheese, but that's the minimal set of ingredients shared by every pizza.

OTOH, cheese might actually be a requirement, can't think of a cheese-free pizza right now. Oh this is going to piss of the vegans.

Things may or may not get more complicated, depending on what you consider Pizza, if you include Flammkuchen which doesn't require yeast or olive oil, and also comes cheese-free by default, instead using some sort of sour cream (and onions and ham).