This one is not likely to be popular here, but I have to be totally honest and say: Miley Cyrus.
Don't get me wrong. I am not really a fan of her or her music (nor am I a hater). But my god does that woman have an incredible voice and a mastery of how to use it.
I'm the same about Ariana Grande. Not a fan, but I saw clips of her singing showtunes with Seth MacFarlane and it was incredible. She has such range and while I'm not really familiar with her music, I feel like her popular songs don't do her any favours.