I'm more into like/dislike than best/worst too, since it's all so subjective. There are plenty of vocalists who don't have the "prettiest" voices, the widest ranges or the most acrobatic techniques, but their talent shows up in their expressiveness / emotion / uniqueness. Here's my list of favorites:
Lisa Gerrard from Dead Can Dance (one of my biggest inspirations)
Bjork (another major source of inspiration)
Erykah Badu
Sinead O'Connor
Annie Lennox
Mariah Carey
Oumou Sangare
Angela Bofill
Esma Redzepova
Gigi (Ethiopian singer...GORGEOUS sound)
Alice Smith (not so much her music, but her actual singing voice)
Diamanda Galas
Skin from Skunk Anansie
Sade
Joni Mitchell
Whitney Houston back in the day
Miriam Makeba
Maddy Prior
Minnie Riperton
Rokia Traore
Grace Jones
Cyndi Lauper (she definitely has a voice one either loves or hates lol)
Tori Amos (another highly contentious voice)
I'm highly tempted to add Prince and Radiohead, since their voices are so feminine...but I won't

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