This list is specifically for plain cheese pizza sold by the slice.
I'm also taking into consideration that it's the apocalypse, so with that in mind:
Lucia
New Park Pizza
Patsy's
Louie and Ernie's
These 4 would be my essential, must-haves. If Lucia wasn't open or was overrun by people trying to have their last earthly meal, Margherita Pizza in Jamaica could serve as a stand-in. But these 4 are worth fighting across the city to get to before it's all over.
Since there's bound to be collapse of infrastructure and potentially unmanageable transit situations occurring, I'll hope that the next 4 are also still available and that I can get to them in time:
NY Pizza Suprema
Pugsley Pizza
J&V Pizza
Sal & Carmine's (ask for it well-done)
But if I'm limited in mobility or access, or some of the above have simply shut down or been destroyed already, my alternates list looks like this:
L&B Spumoni Gardens (square)
Pizza Wagon
Joe's (if Joe is working--his cheese hand is a little heavier than other employees)
___________
Edit: This is based on having made it to 35 of the ~60 places on my list, so I'll update this in the future if things change. Lots more out there I'm still looking forward to!
