As long as we're talking Campo dei Fiori, for cut (hand-held) pizza
you cannot beat Forno, in the corner of the campo behind the flower
sellers.
Yes, ponygirl, we stayed just off Campo de' Fiori a couple weeks ago, and would get cookies and pizza slices at the Forno on the piazza just behind the flower seller as you say, but also a couple blocks away at the original Forno bakery. Tremendous pizza there too, about three different kinds per day.
I've said I want to spend 4-6 weeks one winter in Rome, staying inexpensively near Campo de' Fiori, and Forno would no doubt be part of my daily routine. Cheap eats, as well as a tiny Carrefour grocery just off the other side of the piazza that had everything else--maybe 10 kinds of prosciutto, tremendous mortadella. Salads, everything. Cheap Lavazza coffee for my Melitta filter to make sure I had at least one caffe Americano per day. I could live there cheaper than Florida or Arizona, I guarantee!