Katy's lack of vocals take the cake in contrast to everyone else, but overall Mariah > Bey > Christina > Adele > Miley > Rihanna > Britney > Kesha > Katy (based off who is on the list)
I want to say Kesha, but what she serves can't really be considered "vocals", can it? She does more talking than singing. When it comes to actual singing, it's Katy without a doubt. I've never known a more inconsistent major popstar when it comes to performing and singing.