Although there are a wide variety of ratings and recommendations a hospital can receive from various sources, there is no industry standard for rating hospitals in Thailand. Those hospitals most often considered to be "5 Star" are the ones that meet or exceed international standards and have been the most successful in attracting patients from around the world.
The Samitivej Hospital, Bumrungrad Hospital, Bangkok Nursing Home Hospital (BNH) and Bangkok General Hospital (BGH) are the ones most often considered to be in the "5 Star" category. BGH also has satellite hospitals in the popular tourist areas throughout Thailand, like in Pattaya and Phuket. I'm sure that there are a few other hospitals worthy of being in this category also, but these are the ones mentioned the most often.