I was in Thailand last December (23-31). From what I understand, this week is the PEAK of the peak season, when some hotels are charging their maximum rates, and are more likely to be full.

The weather was very comfortable (BKK/CNX) and although there were lots of tourists around the major sites, nothing was so overcrowded to be a hassle or bothersome.

So I would probably choose any week other than the last week of December.