The little place outside Boardwalk is pretty good and inexpensive. Some people rave about Via Napoli in Epcot, but it's never impressed me that much .. the pizza is pretty 'blah' and is super expensive..
Best pizza for me in the parks is the slices available at Monster Cafe at USF. It's pricey, but they are huge and good. I think it's Pastamore in CitiWalk that has good pies/slices as well. I haven't tried it myself, despite being a pizza fanatic, but many praise the pizza at Louie's at USF as well.