Ok after researching I think I have it down to two:
Tiger Muay Thai in Phuket
: biggest one with most participants, in Phuket, which is further away from Bangkok and seems quieter
Kombat Group in Pattaya
: seems more hardcore, results may be better here, just outside of Bangkok, so potentially not as serene a setting as TMT
They seem comparable in price.
Anyone have any experience or input?