This is a tough one and I can't pick just five. I'll take almost all the ones named so far, plus
Bonnie Raitt
Chrissy Hinds
Ella
Joni Mitchell
Emmylou Harris
Chaka Kahn (sp?) when she's singing backups for Stevie Winwood and Ry Cooder
And that amazing woman who improvised so phenomenally in Great Gig in the Sky on Dark Side of the Moon.
John Hiatt has some incredible female backup singers, but I've never seen their names on any of the credits.