16. At 15,000 square feet, this six-story Upper East Side mansion is something out of a Gilded Age storybook. It's being sold by real-estate developer Keith Rubenstein of Somerset Partners.
Price: $84.5 million
Absolutely every detail is accounted for in this house, from custom marble and woodwork to steam-resistant mirrors in the bathrooms and a temperature-controlled room for storing fur coats. The dining room spans one full city block, and a two-story staff suite ensures that there is plenty of room for the help.