The Frank Stella’s are the best. They are all over the city and they always look amazing. Two of my favorites are the Saatchi and Saatchi lobby (pictured) and the Citi Corp building in Midtown.
NYC's skyline has been drastically changing, and Frank Gehry has added a real masterpiece with 'New York by Gehry at 8 Spruce Street'. It is subdued compared to Gehry's other work, but also elegant, subtle, and delicate. When you stand directly under it the skin of the building can best be described as reptilian.
When I first visited NY in 2010 the New Museum left a permanent impression. I love the architecture and the concept of the piled-white-shoe-box look-a-like building at the edge of the Lower East Side neighborhood. The exhibition program is appealingly different, and the bookstore in the foyer is not to be missed!