Katy, Christina, Kesha, Britney.
Kesha has had good live moments and really bad ones ala SNL.
Britney hasn't sung live except for once in a blue moon since about 2003, and even then her voice wasn't anything exceptional.
Katy really can't stay in tune for a majority of her performances, acoustic sets included.
Christina strains too much and leaves her with a coarse tone throughout her range.
I'm not shading, I'm just saying it how I see it