A few. I left off my choices that have already been mentioned. As has already been mentioned, hightly subjective, which is why I can't name just one.

Esther Ofarim

Nana Mouskouri (whoops, I see she was named by Alice ~~ too late, already on the list)

Judith Durham (yeah, I know ~~ already mentioned, but I couldn't resist)

Marie-Michèle Desrosiers

Annette Hanshaw (when not copying Helen Kane)

Mary Hopkin

Paddie Bell