559. Sia

Australia's Sia was an anomaly as she did not hit it big until she was in her late 30s, a next-to-impossible occurrence for a pop star.  Sia was mostly in the background, though she finally broke through with her sixth album (1000 Forms of Fear), and her artistic style and booming voice lent itself to electro-pop, EDM, and any style of contemporary music.  Had she broken through earlier in her career, the Hall would be far more likely.