Hello,
If you want a stay close to everything, you should go to Fortyseven. You can do everything on foot. However if you do not want to walk, there is a metro station close to the appartement (Keramikos), three minutes on foot. You can reach Keramikos easily from the airport. In the direction of the airport, the following station is Monastiraki, the centre of Athens. The station after Monastiraki is Syntagma.
In addition, the team is very helpful, special thanks to Valentino and Alex.
Best regards,
Jalel