Nikki is kind of hard to beat IMO. Some others (though not all the same theme) that help me relax are Julie London, Jane Monheit, Beegie Adair, Diana Panton, Emilie-Claire Barlow, Jacintha, Nana Mouskouri, Sinne Eeg, and Victoria Tolstoy. Petula Clark’s “Downtown” is great.
If I’m feeling a need for a little more rocky female vocals I normally listen to the Carpenters, Alison Krauss, Carly Simon, Carole King, Linda Ronstadt, and rarely Celine Dion. Oh I shouldn’t forget Eva Cassidy, she has a stunning voice too.