The Library or the Ritz Carlton are your obvious bets based on those criteria. The Library is cool, modern and centrally located, the Ritz Carlton a bit more of a retreat.
If you are are willing to pay for upgrades, Naka Island is an alternative choice so long as you don’t leave them in the apartment block base rooms.
If you could manage the cat 8 budget, your description screams Vana Belle to me. The JW Khao Lak is the other potential, but I’ve never been there so no comment from me...