I think pizza toppings are a very personal thing, so I can't speak for everyone, but I would have to say... Cheese!
Cheese is absolutely essential, and I am actually quite happy with just cheese. A good Quattro Formaggio doesn't really need any extra toppings if you ask me.
That said, I recently spent a few years in New Zealand, and a popular pizza over there which I had never previous tried here was chicken, brie and cranberry sauce, which was delicious.
I think it all comes down to the quality of ingredients. What I'm not a fan of is getting a pizza from somewhere like Dominos or Pizza Hut - and it being loaded with lots of cheap, plasticky meat.
In that instance I would rather just have a plain margarita. But as I said, it's definitely a personal choice!