I think the question, which is the tallest man-made structure in Greece is clear: it is a radio mast of the Greek Navy at Kato Souli. It is 250 metres high. Source: http://users.ntua.gr/chgantes/files/CV_CharisGantes_short_English.pdf

I seem to recall you posting something similar in here before.

http://en.wikipedia.org/wiki/List_of_tallest_buildings_and_structures_in_Greece

Ηere is a list with the tallest structures in Greece.

I added there the radio mast of the Greek Navy at Kato Souli. Please complete and correct it!