I don't mean who sings the best, but whose voice is most expressive and appealing almost regardless of what she's doing with it.
Here goes:
INGRID BERGMAN. The only Ingrid Bergman movie that I dislike is Indiscreet. What a phenomenal actress, with quite a range, too -- though she did not play a femme fatale, she sort of did in the great film Notorious; and then there's her brave and thoroughly womanly missionary in The Inn of the Sixth Happiness (if you tried that one now, she'd have to be a feminist, and that would ruin the deep friendship she forges with the mandarin (Robert Donat); and her distraught but loyal woman with a boundless capacity for love, in Casablanca. What a voice -- what an accent.
CLAUDETTE COLBERT. Marvelously expressive and rich, both when laughing in a screwball comedy, and when weeping ... See her as the chief prisoner of war among the women, in Three Came Back, opposite the great Sessue Hayakawa.
GRACE KELLY. A tossup for Best Cool Voice, between her and EVA MARIE SAINT. Hitchcock did like those blondes.... What a blockhead Ray Milland was in Dial M for Murder -- who the heck would want to bump off Grace Kelly?
GREER GARSON. Her voice is so low, she sounds too old for Elizabeth Bennett in Pride and Prejudice, but for sheer passion and power, she's hard to beat. In her elder years she did the narration for the animated Little Drummer Boy. (And now that I think of it, how in the world did I forget about JOSE FERRER for an honorable mention on the male side? And he too could sing.) In the same category with Greer Garson are ANNE BAXTER (more breathy) and ROSALIND RUSSELL (whose voice was so deep, when she was the special guest on What's My Line, the panelists thought she was a man, till the audience's laughter gave it away).
AUDREY HEPBURN. My pick for the best "pretty" voice, girlish soprano. She wasn't related to her namesake Katharine, about whom Dorothy Parker said her acting ran the gamut from A to B. Audrey Hepburn should have been born twenty years sooner. You could say the same thing about a lot of the actresses who hit their prime around 1960.
DEBORAH KERR. Rich feminine voice ... and she did not have to press hard with her acting. Imagine if movies had not gone tawdry in the sixties, what kind of mature career she would surely have had. And she could sing, too.
Honorable mention: Maureen O'Hara, Beulah Bondi (for Best Old Lady voice), Veronica Lake (for Sultry as Summer on the Bayou voice), Ginger Rogers (tied with Betty Hutton for Best Tomboy voice), Ethel Waters, Vivian Leigh....