Kefalonia for sure, Skala is a nice area, Mounda is a fantastic beach, and Marina Bay Studios are really nice.

I would put the order of preference as follows:-

Santorini
Kefalonia
Crete
Halkidiki

In the case of Crete I would prefer the south coast resorts. Places like Rethymnon are okay for a couple of days visit but given the choice I would go for Chania rather than Rethymnon.

I quite liked the look of the Chania area of Crete myself, but being from Wales can't find a local airport (Cardiff or Bristol) that flies here. They only use Heraklion. Flew from Gatwick to Dom Rep last year but I'd rather a local departure this time.

I stayed at Stavros earlier this year about 10 miles north of Chania on the akrotiri perninsular. Great beaches around the Stavros area, tavernas not expensive and food was generally very good. Eating out in the villages we never paid for a desert, and were treated to a glass of raki at every taverna.
Chania was more expensive but left Rethrymon standing for character.
Chania itself was not as expensive as Rethrymon where the harbour area was a nightmare for restaurant waiters hassling you as you walk through.
The resorts between Rethrymon and Chania had some very cheap prices and are well worth a visit.

I will be flying to Heraklion myself first week in October for two weeks at Myrtos on the south coast of Crete. I have visited Rethymnon and Chania on trips from other resorts but I would not want to spend more than two days at either. Given the choice however it would be Chania in preference to Rethymnon every time.

Sigh, Myrtos....have a raki at Ermioni's for me!

I used to work in Rethymnon and althought it was a lovley resort, it is very stretched out so you can be a long way from the main town. Chania seemed prettier and would be my preference over Rethymnon but unless I could fly to Chania military airport I wouldn't go due to the long tranfer from Heraklion.

Kefalonia is lovely. We stayed in Lassi but it was very quiet. We were still in the pool or the beach at 7pm and everyone else was on their way out. By 9pm most people were heading back and we were the last in all the restaurants. Dramatic scenery and well worth a visit.

Santorini I have only visited on a day trip. Very much a romantic couples island I think and not keen on black sand on the beaches which puts me off.

Hope some of this helps.