I had an amazing backpacker holiday at Railay Beach some 25 years ago. Think the place is completely different now.
Would like to stay somewhere comfortable with the teenage dc, not too overdeveloped for tourism - if anywhere like that still exists in Thailand. And definitely NOT near any all night beach parties. We are planning to around xmas time.
A year or so ago we went to Ao Nang in Krabi and stayed at the Amari Vogue - far enough out of Krabi to be restful and away from the masses, but a short taxi ride to restaurants and the fabulous food night market. We took boat trips to nearby islands for snorkelling and swimming and it was a fabulous break.
Indeed, the food was fab and they were (as everyone is in Thailand) absolutely brilliant with dd. We stayed at Long Beach Chalets on Long Beach too, not as undeveloped as the far south of the island but the beach is beautiful, the food is amazing and the people that own the place are the nicest, most friendly hosts you could ever hope to meet. So much so that we're going back on Wednesday (yay!). Disclaimer: I don't work for Lanta tourism, but I probably should!