In terms of vocals (based on the ones you have so far:
Ariana > Demi > Kelly > Adele > Tori > Kylie >>>>>>>>>>>>>>>>>>>>>>>>> Taylor (lmao is that a joke?)
From those Adele has the best voice no question. Ariana, Demi, Kelly and Tori aren't far behind though. But Lady Gaga by far has the best vocals in music this decade and even Adele can't compare with some of the notes she can hold and styles she can do.