Visited yesterday Taf coffee in Omonia area. It is really a small outlet in comparison to the fame it has.
Taf is reputed to serve the best coffee in town and a cup of cappuccino costs only 2.20 Euro (and a bit more for special varieties). You might say that what is the best depends on everyone's taste and I do agree with you, but in this case there is some foundation to the claim; that Taf is not just a cafeteria, but also a coffee producer and whole-seller. If you are interested, go and dig in their website for more info.
You might have seen the logo elsewhere in Athens. It is because their coffees are being sold at other cafeterias.
We had cappuccino and it was indeed good.
Another of the coffee lovers' cult-centers is Tailor Made in Platia Agias Irinis, but it is often so full that you cannot find anywhere to sit down (it is because Tailor Made attracts even those who not interested in coffee). Taf instead is much more low-key and better for those who want to enjoy one's coffee more quietly. Note, however, that the indoor space is very limited.
Emmanouil Benaki 7-9, Omonia, Athens
Εμμανουήλ Μπενάκη 7-9, Αθήνα
Tel. 210 3800014
Closed on Sundays and Saturdays until afternoon.