Κωστας (Kostas), 5 Pentelis (take Metropolis from Syntagma Square, turn left on Pentelis). Kostas was recommend by a local as "the best Souvlaki in Athens." We went twice while we were there, and the simplicity of the location let the quality of the food shine on its own. It was delicious, and cheap compared even to other Souvlaki we had.
