The absence of Billie Holiday stuck out at me, too. the absence of Ella Fitzgerald is absurd. Useless list.
I'm not even a huge fan of Holiday's music but the quality of her voice is undeniable. My tastes run more to Lisa Gerrard, who I mentioned in another reply on this thread, Loreena Mckennitt (a hugely underrated vocalist and musician) and if you can put whacko-politics aside, I think even Sinead O'Connor deserved some consideration for this list. Like many here, I think the classically trained voice of Pat Benetar certainly deserves inclusion on this roster.