Weather in Greece.

Greece has Mediterranean weather, similar to southern Italy or Southern Spain. "Best" is relative. Some people like it hot, others don't. Summer is hot. Winter is mildly cold. Mid season is in between. Rain more likely in Autumn. Any weather website can show you the monthly averages.

Are you asking about Greece per say or the islands? Personally think mid Sept to mid Oct is best. The Nat Geo look great but expensive.

I have had very pleasant strolling weather and wildflowers in early spring, but swimming is better in fall, when the water of some protected coves and shallow beaches has been heating up all summer.

My choice is LATE May thru mid-June. Yes the water's brisk, but definitely swimmable -- and I'm someone who won't go unless she can swim, not just dip a toe in. THe other plusses of this time vs September (which yes IS nice):

(1) the rains are over, it's sunny, but the hillsides are still green, in some places lush; by Fall, 4+ rainless months = dry, brownish
(2) May-June gives u More Day for your money! Sun's up until 8:45 or later, whereas in SEpt, it gets dark a bit after 7 pm.
(3) In May-June, Greeks are vitalized, energetic, getting boats, towns, villages, painted & refurbished, but no crowds, so plenty of time to chat. By September, after the crush and rush and 'mob scene' of High Season, Greeks in heavily-touristed areas seem a little bit "burned-out," ready to close up shop so they can put their kids in school and get on with their lives. The weather's still fine, but (from my experience traveling at both times) in Sept. one can get that feeling of "the party's over, it's time to go."

I'm with Janet all the way, May/June ist perfect in my book. Greek spring is gorgeous! The water is definitely warm enough to swim. If you like your water as warm as possible, choose some of the most southern islands. Crete is quite close to Africa after all.