I would say Mort, which was supposed to be a traditionally animated movie directed by John Musker and Ron Clements. The official synopsis was: "Mort is a young man who Death whisks away to train as his apprentice. But on his first solo assignment, Mort prevents the assassination of a princess, threatening the flow of history and the fabric of the Discworld."
I just think the concept of Mort sounds very unique and unlike anything, Disney has done before. It might have also had a much appreciated dark edge to it. I feel Disney plays it too safe nowadays. I want more creative risks taken.
I also think the concept art for King of the Elves looked intriguing. But it's easy to build these abandoned projects up in our heads. We don't know how they would have worked out.