Selena Gomez. After seeing those Live Lounge videos, she still can not sing and has almost no power to her voice.
I totally agree with whoever said Demi Lovato and Alicia Keys got worse. Alicia had a beautiful voice in her first two albums but I think she used bad technique and it deteriorated because in "No One", she's off pitch so much and it makes it almost unlistenable.... Demi's tone got weird and she lacks the control over pitch she once had. The only thing that has improved on her is her super high (F5, G5) notes.
Britney and Christina also noticeably aren't as good as they were ten years ago. Britney's voice is more like an autotuned robot and Christina's larynx is probably really tired from being raised every 2 seconds to incorrectly sing (scream) higher notes.
Katy Perry's voice has gotten worse too, she sings super breathy and oddly now. She never had that great of a sense of pitch but I don't care for her "airy" delivery.
