All over the city:
El Almacen de Pizza. It's a chain, like a gourmet pizza hut. Delicious. I particularly like their goat cheese pizza.
Around the neighborhood (nuñez):
Gianna, delicious stone oven pizza, great dough and good prices. Personally I love the margarita and for 70 pesos...can't complain. Stay far away from the one with smoked salmon though, it's revolting.
La Guitarrita, thicker crust than the other mentions but nowhere near 'al molde'. Pizzas are smaller than the massive offerings from almacen but still good for 2-3. They have a great wood oven taste. I usually go for the cantimpalo (very close to pepperoni).