(Scruffy live version of Sunday Girl - so you can hear the singing minus recording production)
I would think that, taken as an all-round package, Debbie Harry - vocalist for the New Wave group Blondie - is probably the best female pop vocalist ever.
Obviously, she was a great natural beauty - which is a major factor in pop. Because of that, not many people noticed her singing. But her vocal quality, as a contralto, was extremely good - especially the lowest notes.
And her whole performance was understated, effortless - the song was performed almost straight, without showing-off.
She also had the advantage of a lot of superb original pop songs to perform, since Blondie were the most lyrical and melodious of the major US bands of their era.
You could find technically better singers (like Aretha Franklin) - who can do all sorts of pyrotechnics; you might even find a few that are better looking - depending on your personal taste (for me, Debbie from Sister Sledge is the most attractive pop star from that era - but that is no doubt idiosyncratic).
However overall, in sum, Debbie Harry was simply the ideal female vocalist to front a pop group.