After reading the first post, I would say these are the first few places that came to mind for me:
First is Passyunk Ave. in South Philadelphia:
https://www.google.com/maps/@39.9298...7i16384!8i8192
Second, people watching in Rittenhouse Square;
https://www.google.com/maps/@39.9497...7i13312!8i6656
And what's Philly if you don't show one of the incredibly narrow streets:
https://www.google.com/maps/@39.9479...7i16384!8i8192
The Belmont Plateau is one of the most popular natural vantage points to take in the skyline:
https://www.google.com/maps/@39.9907...!7i8704!8i4352