depending on where you're located, ECC is one of the largest international churches in Hong Kong and I attend and also volunteer as a teacher in their sunday school. After researching many others, it is probably one of the most well organized. There are over 200 people that serve in this sunday school for hong Kong island alone.
The sunday school curriculum used is the #1 curriculum in the US, called Group Publishing. highly recommend. website is at www.ecchk.org