186: Underworld Emporium
There are a few places that are called masterpieces of the Underworld by the Underworld residents, examples being the Underworld Elevators which are called masterpieces due to their immense height and unknown connection to the three areas of the Underworld- The Meal Room, City of Sin and The Amphitheater.
Another place that's called a masterpiece is The Meal Room, which is called a masterpiece due to its ability to contain the large population of the Underworld. The reason might seem like a non-special reason for it to be called a masterpiece, but according to one of the recorded books he read, its ability to take all the residents of the Underworld was enough reason for it to be called a masterpiece since none knew how the ability worked.