Worth noting that the King's Birthday Dec-5 is a national holiday and Ko Chang, being a few hours drive from Bangkok, tends to get flooded with Thai tourists. Not just the one day either - anytime around then is busy, but especially the weekends.
Anywhere in the tropics can have some clouds and rain almost any time of year, but December is a good bet for the places on your list. Just avoid the lower gulf at this time (Samui etc) for "guaranteed sun" =)
