In Bangkok I tried the Bumrungrad Hospital
https://www.bumrungrad.com/. The hospital has a dental clinic. However I found out from a Thai friend that they are very expensive. I wouldn't recommend them because their prices is similar to what I would have paid in Spore. My dentist in Spore might be expensive but he has the latest dental gadgets.
When I was in Pattaya I've tried 2 other dentists. So far the only thing I've done is cleaning(scaling) & it has costs only about 800 baht per session. Since I've started using an electric toothbrush & water pik about 2 years ago I haven't had any serious dental issues & only had cleaning done.
In Pattaya I simply walked in & made an appointment. They can usually arrange an appointment the next day.
If you are in Pattaya, at the junction of 2nd Road & Central Pattaya Rd is a Tops Supermarket. Just walk down Central Pattaya Road(Pattaya Klang Rd), in the direction away from the beach. There is a goldsmith shop at the right side of the road. The the dental clinic is a few doors from the Goldsmith shop, 10 or 15 meters away from the junction.
The other dentist I went to is the "Dental Art Clinic" at Soi Buakao. According to the appointment card they are located at
502/34 Moo 10 Soi Buakhao. The reason I went to the other dentists was because it was nearer to the Central Mall & the hotel which I was staying at.