I'm trying to sell an appartment in Athens Greece and I want to reach a wider audience. What are the most popular clasified ad sites in your country?

You’re asking the wrong audience. We are all Westerners on this site so I doubt any would know of those countries’ classified ad sites.

You might do a search for their major newspapers and then see if those have online classified sections.

You could try Craigslist.org. It has sub-sites for different cities around the world, and you could post to ones in other countries. You might be able to sell it easily or for more money to people in expensive places sick of local prices – New York, London, San Francisco…

You could also try searching Google in other languages for things like “buy an apartment in europe”, which gives sites which try to translate foreign real estate markets for American or UK buyers who would be daunted by local markets. However they are often not about selling without an agent.

http://www.rightmove.co.uk/overseas-property.html
http://european-property.com/
http://www.zoopla.co.uk/overseas/

et cetera

Are you trying to sell your Greek property to Americans? As an American I just googled “retirement properties in Greece” and This page popped up.
I would start there since that’s the first page an American will see if they’re interested in the type of property you’re selling.