Say you're near the Flatiron Building and you want a slice of pizza. Sure, you could go to Rossopomodoro at Eataly (Slice review here), but you just want a single slice, without much fuss, and you're avoiding the temptation to buy yet another bottle of aged balsamic vinegar.
Instead, walk just south of Madison Square Park for a plain slice at Mozzarelli's. It's not going to win any awards, but if crust crispness is your highest priority, this is the slice for you. The sauce is a little too sweet, with a slightly distracting amount of dried oregano. I found the cheese a little chewy, rather than creamy and stringy, but the crust is really the star, with a satisfying crunch that could happily stand up to a few toppings, but is pretty satisfying to munch through when adorned simply.
In addition to the options you might expect, Mozzarelli's carries pizzas made with gluten-free crusts as well as spelt and whole wheat options. If your diet dictates that you skip regular flour, you might want to keep Mozzarelli's in mind.