I really recommend staying at Kostas' place. It's located very close to several bars, restaurants, shops and the subway. The apartment is clean and has two quite modern bathrooms, but the best thing is the rooftop: looking at the Akropolis out of your private (shared with 4 other apartments) jacuzzi is simply amazing - at night the Akropolis is even lit up.
The staff in the lobby is very friendly and helped us buying tickets to a local football game. If they're not at the desk, the team on the AirBNB chat is very responsive; even at night!