In terms of PURE favorites, i tend to favor the older vocalists from jazz and blues such as Ella Fitzgerald, Sarah Vaughn, Billie Holiday, Lena Horne, Etta James, Ruth Brown and Maxine Brown.

For more contemporary favorites, i'd probably go with:
Allison Krauss
Beth Orton
Annie Lennox
Patty Loveless
Joan Osbourne
Kelly Willis
Mae Moore
Maia Sharp
Sarah McLachlan
Vanessa Carlton
Tori Amos
and Nanci Griffith

I guess thats more than 5, but i never was very good at following directions...

T-