To me, the best and most important thing in a Disney movie is the villain. To me, Disney's always been the best at creating some of the most memorable villains in cinematic history.
So difficult to make a top ten, let alone placing on at the very top. That being said, I remember feeling genuinely frightened and disturbed by Medusa from the Rescuers. She didn't just frighten me, but the first time I saw the movie as a kid, I was scared to death and I quite literally cried the entire following night.