Near the duomo, I'd suggest Pizzeria Toto if you want to order and take out. It's not perfect by any stretch, but I think they do a good job. There are two related locations - I'm partial to the one on Via Dante Alighieri, mostly for takeout since they only have a small counter to sit at. You can either get slices warmed up out of the case or order a fresh pizza (which I recommend).

If you prefer to sit down or can wander a few extra blocks, O'Vesuvio (yeah, of Jersey Shore fame) actually makes really good pizza. Better than Toto. It's pizza napoletana, and a good representation that far north. They have the correct oven. I'd go here.