My favorite building in Philadelphia is the Merchant Exchange (conveniently located across the street from my favorite movie theatre, the Ritz Five) on Walnut St in Olde City.
Completed in 1834 and designed by William Strickland in the Greek Revival Style, it was once a lively hub for commerce and finances (it’s the oldest existing Stock Exchange in the U.S.!), but was named a National Historic Landmark in 2001 and now serves as home base for the Independence National Historic Park. An ongoing exhibit of the building’s history on the first floor is open to visitors. Because it virtually stands alone on a triangle, one can admire it from all sides, especially walking up Walnut from Front Street.
It was constructed with good old Pennsylvania blue marble (and Carrara marble columns). The front of the building, which faces 3rd Street, is masculine and stately while the back (at the triangle point on Dock and Walnut) is quite curvy and feminine which somehow achieves a lovely balance, especially with the cobblestone walkway beside it. There is just enough ornamentation to avoid ostentation, including detailed columns, a cupola with a golden weather vane and two lions guarding the back entrance.
If you sit on one of the benches in back of the City Tavern (just a stroll across the cobblestones) you might feel like a time traveler.