Koh Lipe, a tiny tourist island in Thailand. Amazing.
More recently, the second runner up would be Mullet Bay Beach in St Maarten. Was just there last week and was blown away. Soft white sand (although not the most powdery I have enjoyed) and crystal clear water. So much fun snorkeling from the beach. The water was only very mildly salty and didn't leave that sticky feeling on your skin. And it was only a 10-15 min walk from the famous Maho Beach where planes land over your head.
We were on a cruise and ported at Labadee, Haiti just a couple of days later. Had a blast on the beach but the water was saltier (so had the sticky residue) and although clear, was not as crystal clear as St Maarten.