Hadrian’s Arch (or Gate), is below Parthenon Hill, right before you enter the Temple of Olympian Zeus (Olympeion). Standing on one side of the arch, you can see the Parthenon and Acropolis through it—a really beautiful vista if you can get it. It’s included in the combination ticket for the major archaeological sites of Athens.
Location: Syntagma The Arch of Hadrian in Athens, Attica: The Arch of Hadrian is a triumphal arch which lies in a walking distance from Syntagma square, right between the rock of the Acropolis and the temple of Olympian Zeus. On the northwest, this monument faces the Monument of Lycicrates in Plaka. The construction of this temple started in 131 BC to celebrate the arrival of the Roman Emperor Hadrian and to thank him for his benefactions for the city. Source: www.greeka.com
The arch is an arch, but it's right next to other ruins, so you should definitely swing by.
A bit trick to photograph but the cool thing about Athens is that there is a photo op around every corner!