A
ten.
Why?
1) Can't dance and what she sings live is a complete mess.
2) Most of her hit songs are childish. Firework and California Gurls are

3) She's at her peak and she still doesn't have a true iconic image or sound.
4) Panders to her tween fans too much.
5) No stage presence. At all.
6) Delusional. She tweeted that she thought 'Teenage Dream' was a bridge between pop fans and indie fans / hipsters

And don't get me started on the whole blasphemy Alejandro thing.
The sooner she's gone (and I think she will be gone), the better for the pop industry.