I think most of the islands can be a bit iffy at that time of year. Crete is the furthest south so I guess that would be the best of the bunch. If rain would spoil your holiday I'd try somewhere else.
We've been to Cyprus, Tunisia and Malta in October and they were all fine and dry.

The further south you go the safer you will be regarding the weather.
Crete,Santorini maybe Cyprus will be your best bet.

Been to Rhodes and Santorini in October, apart from arriving in rain to Rhodes, weather was great, in mid to late 20's, the islands are quieter but still beautiful ....and going to Kos in October this year.

Have been to Samos in October.
Weather was excellent for the entire holiday but it got a bit windy and cool in the evening.

October is very difficult to say about the weather, but do know that it is usually the rainy season in Corfu, only been there twice Sept/Oct first time great second time quite a bit of rain and taverna's started to closed a couple of friends who landed the day we flew home said they had rain veryday for there 2 weeks holiday, but have had 1 October in Kefalonia the weather was just like it had been in July, but that was exceptional.
pojo

Went to Zakynthos in October, had 2 glorious hot sunny days, the rest of the week was either dull or rainy, not cold, though.

been to Kardemena, Kos in October on many occassions only ever had one evening of rain in all the weeks we have been. Its a lost quieter this time of the year as the "louts" are not so much in abundance.

Later in October in get can very quiet as the shops and some Restaurants are closing as its the end of the season. Having been other times of the year this is certainly my favourite time to go.

The link below gives historic weather reports for Kos and any other destination you can thing of, might be useful. Points to Kos at the moment but you can do a search.

http://www.wunderground.com/global/stations/16742.html

Mark

Thanks to everyone for the replies. Not yet booked anywhere for October but may look further afield.

October's nice in Kos as it's abit cooler at night.. there was no rain at all...