I love powerful voices, so....
1) Linda Ronstadt. For me, Linda was right up there with Stevie as the voice of west coast chick-rock.
2) Barbra Streisand. Kind of surprised to find myself picking Babs, she had tons of (mostly forgettable) mainstream pop/easy listening/movie tunes, but still, one of the purest, most powerful voices EVER. When she made the movie 'Yentl,' there was a scene where her beloved father had just died and she's singing a song for him -- I cried my eyes out in the theater, and it's still one of the most powerfully beautiful things I've ever heard (and I still get teary).
http://www.youtube.com/watch?v=QwCPAo5e_F8
Such a voice, so much emotion.
(You can see why I'm not impressed with today's over-costumed and under-talented little girls. I remember when our chick singers had voices!)
PS: Honorable mention for Aretha Franklin; remember her string of hits and her great voice? How bout some
RESPECT! 
And what about JANIS!!!!!!! -- you can thank Southern Comfort and Marlboros for that great voice.
EDIT: Okay, Aretha and Janis are really 60's, and this poll is 70's. I guess I got carried away with old memories......
