Road King: I’m surprised no-one has mentioned the Peninsula in BKK. On the ‘wrong’ side of the river, but all the better for that, since the views of ‘downtown’ BKK, especially if you can blag an upper floor suite, are breathtaking. String trio plays twice-weekly in the lobby, the three-tiered swimming pool is a model of taste and refinement, while the outdoor dining terrace is splendid. Smart pseudo-old boat transfers across the river to the Mandarin Oriental/Skytrain pier. And the usual combination of classy elegance and discreet high-tech touches in the rooms.
For something different at the beach, you could try Koh L:ipe, known as ‘the Maldives of Thailand). I agree with other contributors that the west coast/Andaman Sea is generally best. Idyllic Resort, Sunrise Beach. Easiest way to get to Lipe is through Langkawi (Telaga Terminal).
Essentially if you choose an upmarket enough place you’re unlikely to go wrong in Thailand, in my opinion. Have fun!