I would say BKK has the most flights in/out to where-ever.
If you stay with DL metal, you will go via NRT and the schedule sucks (for me at least). You land in SE Asia at midnight, and depart at about 5AM.
Another choice would be the CI flights (a DL partner) which leave the west coast at about midnight and arrive early morning in Taipai (also a major hub). This route can effectively save you an entire day of vacation time if that matters.
You can try southern China as a base, I have not done so thereby will not comment.
All in all, it probably does not matter much. Find the cheapest ticket that hits your schedule.