If we're talking a "pure" female voice as opposed to simply an enjoyable one, and we're not restricting it to current and to folk, then my vote would have to go for Karen Carpenter. Her voice was so clear and smooth without any rasps/rumbles/squeaks/burrs/breathiness or any of the other vocal tricks that so may singers use. There are many other singers who'se voices have a character that are very enjoyable and that I would rather listen to, but that wasn't the question.