I'm a little late to this discussion but find Eva to be somewhat incomparable - her voice may not be more remarkable than many mentioned here, but I think it's the combination of style, humility, recording/arrangements and voice that sets her apart.
I think Alison Kraus and Jenifer Warnes are in that league.
"Too Rich For My Blood"
I like Patricia Barber though not the same as Eva, but I find this song mesmerizing. One of my favorites by anyone.
Interesting the mention of Jane Relf (I thought I was the only one). Sadly there's not enough of her on record - just her work with Renaissance/Illusion (and McCarty). I wish there were more. She's one of my favorite vocalists.