I enjoy good female vocals. My particular favorites (this isn't exhaustive, just what I've typed in a few minutes):
Ella *
Judy **
Barbara ***
Dinah Washington
Peggy Lee
Etta James
Joni Mitchell
Eva Cassidy
Melody Gardot
Diana Krall
Beth Hart
Sarah Jarosz
Gillian Welch
In case you need to ask:
* Fitzgerald
** Garland
*** Streisand
Ella *
Judy **
Barbara ***
Dinah Washington
Peggy Lee
Etta James
Joni Mitchell
Eva Cassidy
Melody Gardot
Diana Krall
Beth Hart
Sarah Jarosz
Gillian Welch
In case you need to ask:
* Fitzgerald
** Garland
*** Streisand