Although not the most soulful, Christina can sing. An engineer that I work with did a session with her, and what took her 10 minutes took an entire day for Brittney Spears (who unfortunately represents typical talent level these days).
Since we're talking about women, my two favorite artists right now happen to be Joan Osborne and Meshell. ALL of the young singers today can learn a lot from these two. Just watch Standing in the Shadows of Motown.