It's hard to say where is the best place, probadly it's easier if telling about best time to visit.
It depends on your hobbies, your interest to see where is more suitable for you to go and activities you should take. It depends on your level of budget to find out where you should stay and places is more suitable to eat.
Anyways, in Thai land, I like Chiang mai and Phuket.
You should have the Pad Thai in Thailand.
In Vietnam, I like Hue and Hoi An.
"Pho" noodle soup is famous especially in Ha Noi.
Cheers,