Local place called Imo's. It's a St. Louis only tradition. They make it with Provel cheese (Provel cheese is a pasteurized process cheddar, swiss, and provolone cheese), thin crust, homemade sauce, and square slices. I've never had anything better. If you ever run into anyone from St. Louis anywhere in the world mention Imo's, Toasted Ravioli, and Gooey Butter Cake (all three ridiculously delicious foods that, inexplicably, aren't found anywhere else but here).