The Cyclades Islands is probably the windiest sailing destination in the Mediterranean, particularly during the peak summer months of July and August. The 'Meltemi' wind blows down the Aegean with some force and provides some exciting sailing. Either side of peak, the winds die down a bit and are a bit more manageable.
Sardinia is an excellent destination for the genuine sailor who's keen to get their sails up and do some actual sailing. There's lots of interesting passages to do as well, particularly the one from Sardinia to Corsica, across the Strait of Bonafacio. Sardinia hosts the exclusive Maxi Yacht Rolex Cup in September every year.
The Balearics enjoy good winds and lovely sailing conditions. With both long and short legs, there's plenty to offer the sailor who wants to get some interesting and exciting sea miles under their keel. Evidence of this is the large number of regattas that are run out of Palma every year, including the SuperYacht Cup.
The Tuscan Islands in Italy are a lovely sailing area for those wishing to get around under sail. Not only is there a reliable and constant wind blowing from the north west, but there is also some longer sailing legs available for the keener sailor. In particular is the French Island of Corsica and several southern Tuscan isles.
Greece has the whole spectrum of wind strengths. The Ionian is extremely placid, while the Cyclades are extremely challenging. Sitting right in the middle, both geograpically and in terms of the sailing conditions is the Saronic Islands. The most 'balanced' sailing in Greece, with everything going for it.