Most unusual/mesmerizing female voice.

Dame Kiri Te Kanawa (more for the mesmerizing part of the question than the unusual part).

Kathleen Ferrier (both mesmerizing and unusual despite the ancient mono recording technology).

And a second for Elizabeth's mention of Emma Kirkby.

Regards,
-- Al

Back in 2010 I had cited several classical and opera performers in this thread. In the pop, folk, and jazz genres I’ll add the Australian singer Judith Durham, who was (and sporadically still is, ever since the group was formed in the 1960s) a member of The Seekers, as well as having pursued a solo career.

I find her voice to be both unusual and mesmerizing in its purity and its beauty.

Regards,
-- Al