If you are in Athens for a few days or several months, the kafenio I Fili as to be on your list of things to do, as-well than a visit to the new Acropolis Museum ....
Because you will learn more about Greece here than in any book or museum, and then because the atmosphere in the evening can be extremely warm and friendly. Note that the spirits are cheap, but not the food. Address: Odos Salamina, Metaxourgio