Start on Wooster Street
If the point of the day is New Haven apizza, Wooster Square is the obvious place to begin. Frank Pepe Pizzeria Napoletana, 157 Wooster St, and Sally's Apizza, 237 Wooster St, are both there, a short walk from each other. Pepe's has been serving coal-fired pizzas since 1925, and Sally's is known for charred, thin-crust pies.
Add one State Street stop
From Wooster Square, you can continue to Modern Apizza at 874 State St in East Rock. It's another of the city's well-known apizza addresses, with thin-crust, coal-fired pizza.
Finish downtown
If you still want one more stop, Bar at 254 Crown St keeps the crawl going without leaving New Haven. It's a downtown bar and pizza spot, which makes it a reasonable last stop if your group wants to shift from straight pizza debate to hanging out for a while.
Keep the plan realistic
A pizza crawl sounds better on paper than it feels by stop five, so I'd keep this to three or four places max. The route stays straightforward: Wooster Square, then State Street, then downtown.