Britney: Work ethic.
Marina and Foxes: Nobody, although poor Marina. Charli and Foxes opened up for ha tour and are seeing more success than her currently.
Katy:
Iggy: Azealia
Madonna has no one she considers enemies. She's too busy enjoying life with her beautiful family and enviable career.
There are nasty and unhappy people like Elton John, but not limited to him, that hold a grudge and continually bash her. But she just gets on with life. Kabbalah taught her that.
People like to think Adele and Taylor hate each other even though I'm sure they don't.
I don't think Miley and Taylor get along anymore, not even sure they ever did.