Katy's voice is probably the weakest of the bunch, but it's endearing and unique to her and works for most of her songs (similar to RIhanna). Rita Ora is a capable singer, but her voice is not interesting to me in the slightest, at least with the material she's been given.
It's Jessie J for me. I enjoy some of her songs, but every time I watch a live performance she is always oversinging each song to its breaking point and it's just beyond exhausting and irritating.