I have Habib Koite, Lhasa De Sela and Christina Branco and agree with you completely, though I would never describe Christina Branco as plaintive. Her "Corpo Illuminado" and "Post Scriptum" albums are also exceptional. I can also highly recommend Lhasa de Sela's "La Llorona."
Another exceptional voice that probably isn't well known here is the Spanish singer Martirio. Her album "Flor de Piel" is one of my favorites of the last year or so and "Mucho Corazon" is also wonderful. The quality of the recording on "Flor de Piel" is amazing, as well.