Most unusual/mesmerizing female voice.

Natalie Merchant gets my #1 vote!
Irene Papas and her performance on Aphrodite's Child's track Infinity from the album 666. If you have heard this it's unlikely to be easily forgotten. Definitely not my favourite track on the album but unforgettable just the same.
Lisa Gerrard of Dead Can Dance (most mesmerizing)
Elizabeth Frasier of Cocteau Twins (most unusual)
Always liked the ex abba singer with “there’s something goin on”
always seemed eerie to me.

 The slight echo, at 2-3 am, ripped, always was eerie to me.
 Still a great song!