I prefer Chiang Mai to Phuket, the weather is cooler, food and accomodtion is cheaper and not so sleazy. Chay-yai gym is by far the best gym in the North having trained many famous boxers. Its a small gym where you get plenty of attention (5 trainers)www.muaythaicamps.com
Lanna has too many foreigners and therefore you might find trainers a little overworked and disinterested.
Bangkok: Sasiprapa is a famous gym good boxers and manager Mr Thakoon spaeks good English, not sure of the web address.
Pattaya: ISS is small but very good (Peter Aerts trains there sometimes) Sityodtong has a long history and good trainers. Pattaya is pretty sleazy though.
Fairtex has the best facilities but by far the highest prices, i dont think its good value for money.
Plenty of choices but for me Chay-yai Gym is where i got the most attention and learnt the most, also a very friendly atmosphere.