A must-try phenomenon specific to just the few towns on the southern New Hampshire Seacoast and into the north shore of Massachusetts. Thin-crust, square-cut, sweet-sauced goodness. I linked a good Eater article on its history - the only thing I disagree with them on is their categorization of it being “bad pizza.” Just be sure to get the “extra cheese” (a slice of provolone) and eat it hot! The main two spots are Tripoli and Cristy’s.
My personal Rec: Cristaldi’s, a hardly-mentioned 3rd contender that only has one location affixed to the end of a firework shop in Seabrook, NH. They're only open in the summer, but the other two are great and open year-round.