I'd say probably Katy Perry. Not for no reason, but she always sings songs completely out of her vocal range. She should do what Kylie does and recognise her limitations as a singer and get songs to compliment the small range she has.
She always attempt big notes and big songs that she just murders.
Anyway, I always found her completely unlikeable and borderline egotistical, until the Ego that is Taylor Swift somehow became massive. Can't stand the sight of her or the sound of her so-called "music".
Taylor Swift's insufferable, whiny and bitchy attitude made me sympathise with Katy Perry.