I don't know why there needs to be comparisons. Leona can sing, Alexandra can sing, Ella can sing. They can all sing very well.
EDIT - more than "very well", I should say. Leona's performances of 'I Will Always Love You', Alexandra's 'Listen' and then Ruth's 'Purple Rain' were all incredible vocal performances.
And I love Rebecca but the only thing going for her is her tone. Her voice is a bit thin and she can't really do low notes.