So here is an informal list of gyms I'd generally recommend, and a short synopsis why. These are maybe one-month-stay recommendations. These are not reviews, just quick overall impressions.
Lanna Muay Thai has been sold and is currently closed for renovation and then will reopen with the new owner. It's uncertain what it will be called and how the gym will be, compared to the Lanna that I spent 2 years at. Some of the trainers from the old Lanna are opening their own gym in Chiang Mai, which I will have to go visit and then will know whether or not to recommend. Same for the reopening with the new owner.
Lanna Muay Thai - casual in tone, you get back what you put in, female fighter friendly, nice Chiang Mai runs, tons of fight opportunities as you progress, Daeng is an excellent lead instructor, on a classic Thai market soi (small residential road). This is the gym that I spent more than two years at starting in 2012, the gym that supported my passion for fighting and fighting a lot. I wrote about how Lanna is the most fight friendly gym in the world, and I believe that to be the case. Everything they do is to prepare for the ring, whether or not you intend to fight. It went through a rough spell when Andy left, but from everything I hear - and from what I see on my visits when I go fight in Chiang Mai lately - it is back to being quite strong.
New note (3/4/2019) - I'll update this later, but since the new ownership has taken over at Lanna they've completely rebuilt the gym from the ground up. Nook and Kru Daeng (one of the best privates in Thailand) are still there, and the training seems like it's become much more regimented and thorough. On the other hand I've heard that in high-season months it can become quite crowded (like a few other Chiang Mai gyms) and that this can impact the amount of guided work you can get. I'm only speaking from afar, but I'd say that if you go to Lanna you should plan to also take a few privates with Daeng to make sure you get the most out of it.
Sitmonchai Gym - good Thai vs Westerners mix, westerners only spar with Thais, low-kick specialists, Kru Dam is world famous and a keen instructor, a western female manager (good for women), do not clinch a lot, Pee A the owner is awesome.
I've heard one story about unwanted advances on a long term female student, and another of a long term fighter who felt he was pressured to fight, but I'm not sure how to weigh these things in an overall way. I wrote about how Sitmonchai may be the perfect mix of things people look for in a gym: Best Muay Thai Gym in Thailand?
Kem Muaythai Gym - clinch heavy, gorgeous mountain location, run by a great fighter in Kem, access to both MAX and Isaan festival cards.
As much as I wrote that Sitmonchai Gym may be the best in Thailand, that was before I visited Kem's high up on a mountain near Khorat. I call it the Shaolin Experience. Big beautiful resort like grounds, grueling training sessions, lots of active fighters. I wrote about the gym here: Kem Muaythai Gym: Hardcore, Beautiful, Clinch Gym Note (added 3/4/2019) Yodwicha, a wonderful clinch fighter turned top International kickboxing fighter and excellent teacher, no longer is at Kem's. He's relocated to Buakaw's gym in Chiang Mai. I would also give a slight warning that one of the krus at this gym has received a few complaints, so if you go make sure you communicate everything with Kem's wife Mo, who is amazing and receptive. Kem himself is one of the best instructors in all of Thailand.
Sangtiennoi Gym - about as close to a "real" kai muay readily integrating westerners as there is, led by a Golden Age Legend in Sangtiennoi. Lots of clinch, hard sessions, rural location, an incredible amount of farm animals.
I wrote about Sangtiennoi's Gym here: Tough, Traditional Muay Thai with a Legend. I found him to be a generous and excellent instructor, and I loved how he ran the sessions with a stick in his hand. The western fighters were not too plentiful, and felt integrated into everything. It has strongly retained its home feel, and honestly, Gigi the giant pig was incredible. Lots of westerners who formerly trained there consider themselves part of a family.
Attachai Muay Thai Gym - this is a new gym in Bangkok, and has to have the most beautiful setting of any gym in the city. Not even close. It's run by a legend of Muay Thai, Attachai, who has returned to Thailand after many years as a trainer at Evolve in Singapore.
I trained with him in private he has an incredible way of teaching timing, and Muay Thai response, which I've never seen before. Everything had a mix of semi-sparring to it. A really interesting teacher. I sent Emma Thomas over to the gym - she had been looking for a new gym in BKK for more than a year - and she fell in love with it. This was my initial review.
Hongthong Gym (Chiang Mai) - My private with Joe Hongthong was absolutely wonderful. He thinks creatively about the fighter I am, and then about how to enhance that. The brothers that run the gym each have a different emphasis, Gen is Muay Femur, and Joe is Muay Khao. They've had successful women fighting out of their gym, and from personal experience I'd say that if you are a Muay Khao fighter Joe would make a wonderful teacher. [edit: The new location of Hongthong Gym is pretty spectacular, check out the video a few weeks after opening - I've moved this up into my main recommendation section because the facilities have dramatically improved, with the same quality training.]
Manop's Gym (Chiang Mai) - For those that want a gym that is a bit more personal in their training Manop's new gym in Chiang Mai is definitely something to check out. Manop is famously known as Saenchai's Yokkao trainer, and he's left Yokkao now to start his own life in Chiang Mai. He is incredibly perceptive as a teacher, very, very technical. The gym is in a quiet neighborhood outside of the city, and seems like a great opportunity learn and train hard.
One of the more difficult communications I receive is a request that I recommend gyms for other people. At first blush this seems like an obvious request. I've trained and fought in Thailand for nearly 5 years, I'm well connected to other fighters and serious students in the country. I'm pretty forthright in my opinions. But aside from the fact that I have a really full plate training, fighting and writing, the simple truth is that it is very, very difficult to do this kind of recommendation - I often equate it to recommending a college to someone - so many factors go into this. Not only are everyone's needs quite different, and most of those needs vastly different from my own, people often are looking for a combination of factors in a single gym that is almost fairytale land: very cheap, "authentic" and not many westerners, but also very "technical" and investing a lot in explanations and teaching, and treating you as a valued customer.
I'll add to this that I have only trained long term in a few gyms in Thailand - though I've taken privates in many - so my first hand long term experience is actually quite limited. I have heard a lot of feedback from others, so in a certain sense I am informed, but not definitively.
So I'm starting this thread just as a place to list gyms I feel comfortable recommending, and some of the reasons why. I also in the following reply will add gyms that I've heard things about, but am not well-informed on. I totally understand that choosing a gym is one of the biggest decisions one can make in your trip to Thailand. The wrong gym can be a waste of a "once in a lifetime" experience, not to mention a chunk of money. I won't go into depth, but perhaps as the thread grows, and questions get answered the thread becomes a resource to many. The one thing I would say is that if it is your first time in Thailand don't pay for a big package in advance. Go to the gym you think you'll like, for a few days or a week, and then if you do like it, consider staying longer term. I also advise seeing at least one other gym, even if you like the first one, so you have some perspective. In our first stay (2010) we had a great time in Chiang Mai at Lanna, but then we went down to BKK trained at another gym (Sasiprapa) for an equal amount of time. We ended up liking Lanna better, but it was totally worth it to do both.
Please post all gym recommendation questions you have for me here on this thread, and not on Facebook, YouTube or Reddit. That way the conversation can develop and benefit others too! (This list and its descriptions will be revised over time)