The most important element in a story depends on what story you are writing. More to the point, what genre you are writing in.

Detective stories require wonderful plots, as do action-adventure and spy stories. Romance requires wonderful characters. Sci-fi/fantasy/paranormal require wonderful and unique worlds that are different from ours.

So first you think about the genre you will write in. Then you know what’s most important and that’s a good starting point for a great story.