I've been to Tioman, Krabi, Phuket and Koh Phi Phi, but I think the best one was Bamboo Island off Koh Phi Phi (pictured above and beside). Very relaxing ambiance, clear waters and blue skies, willowy greens and soft sand with lots of shells and corals to pick up and admire.
I would have to say a resort in Thailand, though it was years ago and I cannot remember the name of the beach. It was not the main tourist one and had lovely sand and beautiful water as do some of the Asian beaches.
When I visited Bali last year I stayed at Seminyak, and the beach there was absolutely beautiful. It was wide and golden, and close by there was an ancient temple, Pura Petitenget. One evening, while I was down by the beach as the sun was setting, priests from the temple were holding a traditional ceremony down on the sand. It was absolutely magical!
We were just in Phi Phi but didn't go to Bamboo :( - we did have a magical hour in Pileh bay, but there is no beach....I liked Tioman, but without the sandflys, and Lankawi is lovely. I'll say Tanjong Rhu on Langkawi, because of the big limestone outcrops...