I've been to Thassos and Lesvos (both over 10 years ago, I'm afraid) and Paleohora (about 3 years ago)

Structurally, Molyvos is the most attractive, with stone houses and little alleyways and the castle - plenty of places to eat and drink - but th beach is poor. Petra WAS lovely, but this was 19 years ago - so I can't comment on how it's developed over those years (I do know that the taverna I used to eat in is now a bar, so I'd guess that there's plenty to do) - good beach.

Thassos Town itself isn't very attractive, but its surroundings are - pines, old ruins and glorious beaches.

BUT - Lesvos and Thassos are in the north of Greece and the weather can go downhill quite early on and get quite cold at night.

Paleohora isn't terribly attractive but it's got a great feel in the evenings when the main street is closed to traffic and all the bars and tavernas get their tables out. Choice of sandy beach (windy) and pebble (very big pebble) beach - not windy. The surroundings are glorious and you can go to Elafonissi by boat for the stunning (if crowded) beach there.

Hope this helps!

sorry the post is late (blame it on the postal strike)
like Elenitsa I went to Lesvos 10 years ago. Molyvos was and I believe is still lovely but I agree the beach is poor. I went in Agust and can say I have never been insuch cold seawater. Ever!
Went to Paleochora three years ago (twinned it with Georgiopolis), of course it's further south and whilst there are no guarantees it should be warmer. We liked pebble beach and the water was excellent. The eating out was varied and tasty. Cannot comment on the other places sorry.

We have been to all named destinations and just returned from Lesvos for the 2nd time in 3 years. There are some excellent beaches on this island but as stated not in Molyvos. Paleochora is slightly remote but does have a long season and very much enjoyed Thassos (Skala Potamia). For us Lesvos wins.

Lesvos was very busy 10 years ago. We were there as well, staying in Molyvos (and enjoyed it).

Lefkas is a yachting island (or nearly an island) and we stayed in Nidri which is well worth a trip.

My favourite of the three is definitely Thasos. We are not really into beaches and the archaeology, the walks and the tavernas suit us perfectly. We have stayed on or near the main harbour both times and loved the main town. It is a working island with the splendid marble quarries. Buses around the island. We shall return.

Hi
I have been to Kefalonia, Thassos, Skiathos and Lefkas, however I have not been to Lesvos so will give my opinion as follows:

Like you, loved Skiathos, the beaches and the town in the evenings so in comparison to Skiathos . I found Kefalonia very quiet. We stayed in Lassi which is supposed to be one of the busier resorts (my boyfriend and I were in our late 20's) and had to walk into Argostoli most evenings for some life. We found that most people were heading out to eat about 7pm while we were still in the pool or sitting on the balcony. We went out about 9pm and most taverna's were empty and there was not much to do after we ate.

Loved Thassos, nice sandy beaches and Thassos town was lively enough for us with plenty to do and plenty to see on the island. The only downside would be the weather in September as this is the most northerly of the Greek islands. We had a few massive thunderstorms when we went in August and nearly missed the ferry back to the mainland!

I have just returned from Lefkas and found it to be a much underrated island. A very dramatic island with amazing beaches and sea the colour from brochures. Possibly the most dramatic beaches and azure colour sea I have ever seen on a Greek island before. The only down side is the island is quite big and the beaches on the west side take 1 - 2 hours to reach down very winding mountain roads. (One beach has 400 steps to climb down to so prepare for the climb back up!) You definitely need to hire a car or a moped to get to the lovely beaches. The east coast beaches are narrow shingle with the main road running behind so not the best.

Nidri had everything you could wish for and was very friendly. You didn't feel pressured into eating in a particular restaurant and could wander up and down the harbour front looking at the menus at your leisure. Although Nidri is quite commercial, the harbour area is closed to traffic at night and is great for people watching. September should be quieter anyway. You can take a boat trip to Fiskardo, Ithaca, Meganissi and around Skorpios the Onassis island. There are waterfalls to visit in Nidri and sailing boats to hire. It has the best of both worlds being linked to the mainland so you don't have to get a ferry but it still retains the island feel. I'd definitely recommend it and will go back to Lefkas again.

Only my opinion, hope it helps!