Going back to the 70s and 80s, Flora Purim was incredible, with energy and range that would not quit. I see her and Airto every time they come to town, and over the last aprx. 10 yrs, she's had trouble with her voice. The last time I saw her, about 6 months ago, I was very pleased to hear her back in very good form.
Tori Amos...awesome, wild, an amazing composer too. She goes from subtle to dymanic in the blink of an eye. Amazing textural qualities to her voice. I like to listen to her simply musically, not usually wanting to get into the words or meaning of the songs. Give some credit here to Kate Bush, who innovated much of this style.
Amanda McBroom has a beautiful voice. The song 'Amanda' on "Growing Up in Hollywood Town" is marvelous and truly captures the feeling of the words. I wish she had more recordings out there.
Diane Schuur can really sound sweet, and also really belt it out with stunning clarity. Not really the type of music I listen to much, but once in a while, I just have to hear her sing.
Joni Mitchell is incredible when she slows down and isn't putting so many words in. 'Blue Motel Room' off of Hejira is a great example of her at her smooth, fluid vocal best.
I have to plug for Phoebe Snow. Man, she did some beautiful stuff, and her phrasing showed real interpretive talent. Even that recent TV commercial that had her singing a few lines of Sills and Crofts "Summer Breeze" was captivating.
Maire Brennan of Clannad, who someone mentioned above...I love her especially when she sings those mellow tunes in her native Irish tongue.
You can't pass up Loreena McKennitt...I think she's my favourite, and every bit of depth in her soul she communicates to you, and that's no small amount of depth!
Diana Krall...I know, she's fine listen and her records are well produced, but I still don't think she's pushed herself or her music far enough to be considered one of the greats yet. It will be interesting to hear if she ever does really make the effort to bust out, and offer the world of music something it's never really heard before.