Writing fanfiction for books, movies, and TV series are the easiest because you can follow the plot line of them but also deviate away from them to add your own twists and ideas.
But I think writing original stories, even AU's, are the most satisfying (for me, at least) because it's like, "Wow, that's my work. That came from my brain─all of it." Even if the original story with an AU with an original plot line isn't finished.