Most unusual/mesmerizing female voice.

Billy Holiday in her prime.

I always liked the chick from Sixpence None The Richer also.

Beth Gibbons (Portishead) is another.

And lets not forget Eartha Kitt

Nina Simone, vastly underrated

And now for something completely different

Dame Kiri Te Kanawa (more for the mesmerizing part of the question than the unusual part).

Kathleen Ferrier (both mesmerizing and unusual despite the ancient mono recording technology).

And a second for Elizabeth's mention of Emma Kirkby.

Regards,
-- Al

one last one from the late 60's Beverly Biven's of the We Five