I don't think it's too hard to work out the direction in which the producers are managing the audience to vote.
Songs which provide the contestant with big note moments, songs which provide the contestant with the opportunity to express things which are consistent with Nancy's character and situation = conditioning the audience to vote for that person.
Examples from week 1: I Like Rock N Roll was a clearly favourable song for Samantha to sing. It was irrelevant that it was a pop song; the point is that it gave the fresh-faced 17 year old an opportunity immediately to demonstrate that she could portray the raunchiness or ribaldry of Nancy. Whereas Get Here was not about sex, or craving, or passion - it was an elegant, soulful song. Sarah might as well have sung O Ma Babbino Caro for all the good it would have done her in transforming her into an East End prostitute.
The First Cut Is The Deepest was a double edged sword. It simultaneously alludes to the fact that Nancy is/will be brutalised, but it could also be seen as referring to Niamh being cut from the show. Besides, I am not sure if self-pitying reflection forms the greater part of Nancy's screentime. Similarly, Mad About The Boy makes a superficial allusion to Oliver! but it doesn't really Nancify.
Suddenly I See was a pointless song from the point of view of Nancification, as were most of the others.
I would say that Samantha and then Jodie's songs were the most Nancificatory.