Depends on how much time someone has. If it's just a week or so, I would do Thailand. Fly into bangkok, give that a day or two, then head out to Phuket or Krabi areas.. Also I would go to Railey Beach.. Still one of my favorites and the food in the area was amazingly good, especially the italian restaurant. Can't miss a boat trip to the Phi Phi Island. Still the most beautiful water and area I've ever seen. Makes the carribean look mediocre which is shocking.
If someone had 8-10 days, I'd fit in going up to Chiang Mai and visiting the elephant reserve. The interactions with elephants and the experience of bathing them in a river in such a remote area of the world is unreal.
Now if timeline is a couple weeks lol.. People should fit in Cambodia, go to Siem Reap, Angkor Wat nad Phnom Penh. If budget isn't a factor, then definitely head into Singapore for a couple days. Indonesia, Malaysia, Vietnam, Myanmar, Laos.. All those would be low on the list. Vietnam definitely has some beautiful coastal areas, but still a lot of impact there from the Vietnam war. US passports not loved still in some areas.