If you come just for Muay Thai and don't care about anything else then Sitjapho in Hua Hin(The best for technique), or FA group in Bkk( the best for clinch). But, if you are not a pro and wanna enjoy your life in Thailand more, choose some gym in Phuket or Chiang Mai. There are good gyms every where, the training is usually 2 hours for one session. What are you gonna do the rest of the day?
I would not recommend committing to any muay thai gym for a year. The trainers are gonna get lazy once they know you paid ahead. Also, after you ask for the price, you are gonna get the price for training 2 times per day. You should ask how much for 1 time per day. That's how I always get 40% off. Plus, the gyms that make those types of visas are mega gyms for foreigners. If you wanna learn real muay thai, you should look for gyms that are relying on local fighters.