Hahahaha. Oh boy. Sbarro IS far, far, far from the best pizza ever. The pizzerias that use fresh mozzarella, sweet sauce, a little olive oil and cook their pizzas in a brick oven so the bottom has a nice crisp are the best. There are a few in NYC, so I wouldn't be able to say any particular one is the best. And I've grown up on pizza in NYC and I don't know anyone who would even think about Sbarro if they had a pizza craving. Most local places (but not all, like the disgusting Pizza Mercato stuff) make better pizzas.
And saying that "If their pizza was so good, they'd be international like Sbarro" makes no sense. That's like saying Outback Steakhouse makes better steaks than Peter Luger, Ruth Chris, Del Frisco's, Palm, etc. because Outback is international. Now, a decent international pizza chain (perhaps only in London) is Pizza Express.