Definitely the north of Thailand with Chiang Rai being my favorite. Mostly because I have spent there 3 months working with an NGO working on Children' Rights Development. I just love the people there and I had the feeling that the north is much more relaxed than the south. "Sabai Sabai."
Cannot wait to go back. What did you enjoy the most? Have you been to any other country in SEA?
I haven't been in northern Thailand, yet. I absolutely loved Koh Tao, Krabi was really nice as well. Phuket was okay I guess, but I expected a little more from it.
I've been to China 2 years ago. It's completely different from the Netherlands and was very interesting to get to know. Only downside is that almost no one speaks English.