Jane Horrocks, Jane Horrocks, Jane Horrocks!
Because this actress actually had a play written around her mimicking talents called The Rise and Fall of Little Voice. In it, she imitates Judy Garland, Marlene Dietrich, Shirley Bassey, and others, and she is so frighteningly dead on that when they shot the movie (Little Voice, which I highly recommend), they had to put a disclaimer at the end that she did all her own vocals. The mimicry juxtaposed with her portrayal of the actual character makes for an amazing and seriously inspiring performance.
"Peace! The charm's wound up."
--Macbeth