Per Henry Jenkins, fiction is best understood as a platform: a system to build on. The thing you build can be as narrow as your own interpretation, or as expansive as fan fiction, fan art, movies, video games, or even physically-realized artifacts from the fictional world.
So one way to judge the success of a story is to look at how much additional creativity it inspires. By this measure, Harry Potter is a modern masterpiece, and Shakespeare is the king of all time. Seems about right to me.
P.S. No, I have never actually finished one of Henry Jenkins’ blog posts, either. But the first three to four paragraphs are always super-smart.