Depends on what you are after ?
Gorgeous beaches all to yourself, or with people around ?
I went to Koh Samui and koh tao in Nov06. Maenam beach on samui was beautiful you could laze about in the sea and look across to koh phangan and koh tao in the distance, and we basically had the place to ourselves. But we stayed in beach huts with no hot water, not a hotel. However Chaweng is only 20 minutes away. Its not as bad as people make out,ask your taxi driver to drive through the red light district, now there is a cultural experience for you.
Koh tao was slightly off the beaten track, we were ferried about on the back of a pickup, but once again you get the joy of a longtail over to the "twins" two little islands off the shore, the snorkelling here was amazing, and its so small there are no roads.
Last year i went to the Andaman side. Ko Lanta is beautiful, keep heading south and it just gets better and better, but once again you wont find any hotels down here.
Ko Ngai looks like it is slightly upmarket from the few days i spent down there, and you really get the feeling that you are a million miles away from civilisation.
Slightly closer to the rest of the world, why not try Tonsai beach. There are some very nice bungalows here now from about 2500bht a night. Tonsai was my favourite place in Thailand.So far. It doesnt have much of a beach, but the scenery is just staggering, and the atmosphere is very chilled out.
Basically, pack your bags and get out there i reckon you can fit 4 destinations into 11 days.
Enjoy.
edit: 17th december,high season,expect to pay full price for everything, but the weather should be lovely.