I have been going to Tarntip Massage on Sukhumvit Soi 7/1 for several years now and have no problem at all recommending them.

They are legitimate Thai massage, located on a small street off Sukhumvit Road, said street being admittedly notorious for other things besides legitimate Thai massage.

They're on the left as you walk up the soi from Sukhumvit. Easiest landmark is the sign on the right for the notorious Eden Club: they're across the soi and a bit south of Eden Club.