I'd say....
Kelly - The Day We Fell Apart (still one of her best songs though )
Taylor - Probably I Knew You Were Trouble or Welcome To New York. Or Bad Blood, the background vocals
Gaga - Paper Gangsta (intentionally done for effect though) and Starstruck.
Selena Gomez: Off The Chain, Intuition, Shake It Up, Dices (I'm probably forgetting a few other songs)
Demi Lovato: Work Of Art, Me, Myself & Time or Yes I Am (can't decide between the 3)
Kelly Clarkson: Wash, Rinse & Repeat
Hilary Duff: Probably Chasing The Sun (Probably i'm forgetting a few songs as well)
Bridgit Mendler: Determinate or Breakthrough