Mine was Fraser Island, a small island off the East Coast of Queensland, Australia.
The island is absolutely amazing. Great wildlife and the beaches are so good because they are natural. I live on a beach, and all of the best beach holidays I've had have been natural, uncrowded beaches. They are SO much better. The dingoes were awesome to watch and there were crystal clear lakes there, and massive sand dunes that you could slide/board down into water.
Remembering makes me want to go back...