In my opinion, it’s this room, identified in the floor plan as the “Gallery”:
Why do I see it as the most luxurious? Because it doesn’t really serve any purpose; it’s a glorious waste of space. And in New York City, having space to waste is a huge luxury.

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Also boasting lots of space, as well as ample light and Park views, is the master bedroom:
Just like the library, it also has that “country house in the sky” feeling to it. The same goes for this lovely lilac bedroom:
It looks like it’s larger than the master bedroom, doesn’t it? But it’s not, at least according to the floor plan….