Bumrungrad is probably the best quality, but if you are o.k. with giving up the five stay hospital atmosphere for something that is still better than most dental clinics, Bangkok International Dental Clinic (BIDC). There is a small branch in one of the malls, Emporium mall I think, but I've always gone to the main clinic. It is in a large modern building with multiple dentists all under one roof. You can take the subway to the Thailand Culture Center stop, then it is about a 15-minute walk. Slightly hard to find the first time, easy to find on return trips. Or just take a cab there from near Asok intersection.