The Conrad buffet lunch is also excellent, and quite good value to boot! Occasionally hosted business lunches there, and although the food is international as well as Thai, it was consistently very good.
Added. If you dine regularly with Marriott, the Marriott Gold Dining (in Thailand) is good value as 1 meal is free, and drinks are discounted. Comes with a bunch of accommodation discounts for Thai properties as well.
http://www.marriottgoldcard.com/en/l...php?lang_id=10