I trained at SUN fitness for about 5 months daily earlier this year. Good gym, air con is ok. Good equipment, they also have a gym in naklua you can use. It was not to busy but now it's high(ish) season so I imagine it is busier. As mentioned 900 baht a month.
When I go back in January I will be training at the new Elite gym on Sukhumvit road next to McDonald's. Same owners as Elite gym in samui. Good Aircon, all new equipment. 200 baht a day, 500 weekly, 1500 monthly.
I'd say at the moment it's the best gym. I've trained at Tony's, Castra, Universe, Coco's and fitness 7 over the past 12 years.