Certainly the Oriental is an old favorite, but IMHO I think that it has slipped somewhat. The Peninsula is wonderful but I would suggest trying somewhat a little different. I don't think that I have seen the Sukothai mentioned anywhere but it is a gorgeous place with fabulous service. The architecture is much more appealing as it is not a generic highrise like the Peninsula etc. It has beautiful grounds right in the heart of Bangkok. It is right next door to the Banyan Tree, which is also a very nice place, and the Metropolitan which is newer, hip and elegant as well. Do yourself a favor and try the Sukothai. The restaurant called Celadon is at the Sukothai and is one of Bangkok's best.

Sorry, LUXURY, I missed the mention of the Sukothai and the Celadon in your post.