Gyms that I have heard positive things about, or which I visited and have qualities that might appeal to a certain kind of traveler/student/fighter. These are not gym reviews, just quick impressions.
Sitjaopho (Hua Hin) - This is a gym in Hua Hin that is quiet popular with those looking for "technical" instruction. It has a strong Swedish connection, as well as a following with some from the East Coast (USA). I've have experienced one afternoon session here and was really impressed by the organization and work put in by everyone. Kru F is the captain of the ship and works directly with his students, sparring and padwork and clinching, and he seems to set the tone that is carried on by everyone in the space. There is a LOT of sparring (very light, very technical), padwork, shadowboxing, some students hit the bag but most didn't, and about 30 minutes of clinch (many participating, but not all), followed by group conditioning. Friendly space, dedicated students, Thai trainers and on this day all non-Thai students, but everyone on the same program from beginners to definitely-experienced fighters.
Chatchai Sasakul Gym (BKK) - the former WBC world champion boxer Chatchai is highly recommended if you want to work on your boxing. Precise technician, great instructor. Probably the best boxing gym in Thailand, home of several current world champions. Private sessions are best. You can see a full private session with him here. They also have some nearby accommodation for longer-term stays as well.
Dejrat Gym (BKK) - This is a hidden gem in Bangkok run by the coach of the Thai National Team, Arjan Surat. Watch our session together. It just is a very "Thai" gym, so I couldn't recommend it in a broad way, either in a cultural or instruction sense. It's no-nonsense Muay Thai that is focused on its serious Thai fighters. They have had experience with female fighters. Go here only if you want some sort of immersion, are prepared to work very hard, and be positioned in a traditional hierarchy. Not a lot of English spoken. My session with Arjan Surat: Arjan Surat 2 - His Old School Tough & Defensive Style (94 min)
Burklerk's Gym (Lampang, contact here) - outstanding instruction from a Legend in sleepy and beautiful Lampang. He and his wife have opened up a brand new resort style gym in Lampang. I wrote about his original home gym here: Burklerk's Family Run Gym in Lampang. Burklerk has a beautiful, powerful style and each time I visit I learn things. Even 5 minutes with him is gold. It's a small community gym in a quiet neighborhood, but not a fighter's gym really. Go there for the time with Burklerk, but there won't be much sparring or clinching. My session with Arjan Burklerk in his original home location: Burklerk PInsinchai - Dynamic Symmetry (82 min)
Keatkhamtorn Gym (Bangkok) - This gym is an authentic kai muay gym in Bangkok in that it is still very focused on growing Muay Thai stadium champions from an early age. This means that it is a great gym for small bodied westerners especially those interested in immersive clinch. Immersive clinch the way Thais learned, but be warned it takes a while.They have tons of young male fighters between 45-52 kg, and are a Muay Khao gym, which means that you'll be encouraged to develop proper clinch fighting habits. I will definitely make this my clinch gym when in Bangkok. The owner, Teerawat Chukorn is a Police Captain and very kind, and speaks English. You can contact them through their Facebook page which will respond in English.
PK Saenchai Gym (Bangkok) I have never been to this gym during regular training at all, but it is a favorite of Westerners both who are seeking to train under a big name and those who have been in Thailand for a long while and decide to move over there for the fight opportunities and training alongside contemporary stars of Muay Thai. A head trainer is Detduang Pongsawang, who was a great fighter in the Golden Age. From what I understand it's a kind of "build a bear" method for training, so you can decide how much or how little you want to do by speaking with the manager and he works it all out for you. He sounds very personable and his English is very good.
NungUbon: (Ubon) this is a lovely little gym just 10 minutes from the Ubon airport. There are fighter rooms, local fights, and NungUbon's son is a stadium fighter in Bangkok so trips down occur when he is fighting. It's mostly westerners, who for whatever reason tend to be tall, so if you are a larger person and looking for a more rural experience but want similarly sized clinch/sparring partners, this is an option.