Who.
Most people, I believe, will go with “what.” The consensus seems to be that story is all, or at least, the most important element in any work of fiction. But if you think back, what people seem to remember most, the thing that grabs them and stays with them is great characters.
Not everyone could tell you the plot of any of the stories, but everyone knows the essence of who Sherlock Holmes is.
Gandalf and Frodo. Miss Marple. Hannibal Lecter.
Heathcliff and Cathy. Huck Finn. Tom Sawyer. Raskolnikov. Mr. Micawber.
Creating a fictional character who seems not only believable, but more interesting and more real than the actual people we know is what makes books resonate through time.
Of course, as always, I could be mistaken.