Some suggested peer sets for the parts I know, albeit clearly highly subjective -

Above Marriott portfolio: MO, Capella, Siam, FS
St Regis: Kimpton, WA, Park Hyatt, Sukhothai, Siam Kempinski, Sindhorn Kempinski, Okura Prestige, Rosewood, maybe Peninsula at a push (Sukhothai is my recommendation of these properties assuming equivalent status and pricing, for reference)
SGS / Athenee: Anantara Siam, Banyan Tree, Grand Hyatt, Shangri-La (I would take SGS all things being equal as above)
Marriott Surawongse / W / JWM: So Sofitel, Hyatt Sukhumvit, Conrad, Hotel Indigo Wireless Road, Anantara Riverside, IC before it closed although maybe can move up a category post-renovation (I like Marriott Surawongse but there are arguments for some of the others too)