I stumbled across this fruit shake stand on an alley just off D Bui Vien in Saigon (Ho Chi Minh City). You can see the number in the photo(84). Its right in the middle of the backpacker area. The shake was not only the best value, you get a fairly large one for 20,000 dong or just under $1, but it was the best one that I have had in SE Asia. They even put a few chunks of fruit in on the top. I had a mango/avocado and it was heaven. The avocado made it very smooth. Also tried a mango, pineapple, and banana which was outstanding as well.
They also are set up like the Soup Nazi from Seinfeld. A few old ladies run it and they make you sit down on little stools in the alley, write down your order on paper and wait. I saw 20 to 30 people typically waiting, but it is certainly worth it. I almost stayed in HCMC another day just to try the Passion Fruit which somebody told me was fantastic. I think it just depends on whatever you like and they have just about every kind of fruit imaginable.