Pizza in NYC is sort of like cheesteak in Philly... it is pretty good almost everywhere (except chains and malls) You will find pizzarias all over the city and they are mostly good with great selections of slices.
Grimaldis in Brooklyn is the place to go for bragging rights and foodie cred Grimaldi's Pizzeria
this is where natives will wait an hour to get Pizza.
Little Italy is largely a tourist attraction.
The Best slices IMHO will be found in the still ethnically Italian borough hoods. Belmont (Arthur Ave) in The Bronx has some amazing pizza and is in general a paradise of Italian food shopping. It is a bit of a hike for a tourist... but the Botanical Garden is right near by and so is the Zoo so it is a nice side trip although I would use caution near the Tigers if I was carrying a bag full of fresh pastry and cheese.
oddly enough if you are not looking to go off the beaten path Rosa's in the lobby of the Empire State Building is actually very good (This is a working persons luch place not a tourist joint it is tucked away on the SW corner of the lobby near the post office.)