Story retold
The Arthurian legend is probably the most retold story of all time. There might be more popular themes, but they usually tend to be more abstract where these stories usually always refers or claim to be the "true" story. Most of the stories share the same characters, places and events, but the setting can vary widely.
What makes the arthurian legends unique is that it's not quite history, but it's not strictly folklore either. There isn't one original story upon which all the later stories would be based on.