semiviking scratch the waitlist idea ! You need to find out whats available for a group of 20 for christmas/new years in Phuket/samui/Krabi.
If you can find availability then perhaps someone can help with the best from those available.
Perhaps one of the newer resorts on Samui will have availability but the best resorts will be fully booked probably from the previous Christmas.
Good luck and let us know how you get on !