Elite Fight Club is good, but has many Russians last time I was there.
Ingram was strict and tough when I was there last.
Yokkao is kinda a rip off IMO. Sure, you get to train in the same gym as Sanchai and Singdam, but it's filled with fanboys who think they are much better than they really are.
Attachai's gym isn't far from Thong Lo, but I didn't enjoy my training there at all.
Muay Thai Academy MTA is the old Rombo gym?, had a good rep back in the day, not sure about now. Next to the Super Export shop, which has tons of equipment.
96 Penang is not too far away, about 15-20 minutes if you catch the train/subway at the right time.
Eminent air gym is solid also.
These are fitness gyms for rich Bangkok girls. Trainers might be good, but not somewhere I'd train long term.
8 Limbs
RSM
FITFAC
Chakrit
Krudam
Mankong Phranai