My suggestions are:-
Bangkok - Salathip at the Shangri-la, the thai restaurant at the Oriental and the restaurant at the top of the Banyan tree.
Phuket - The Boathouse, On the Rock, Ruam Thep Inn, (these last 2 are at the southern end of Karon Beach) and the Savoey and Ban Rim Paa in Patong.
Can't remember any memorable food in Koh Samui although that doesn't mean we didn't eat well!