I don't think there's any way of answering this except to say it needs to be songs that move her, in some sense. The essence of it, for me anyway, is that the singer approaches it in their own way and to do that, they need some sort of attachment to the song. Otherwise as the hedgehog song of a few decades had it 'You learnt all the words and you sang all the notes but you never quite learned the song.'

I tend to the traditional, personally, so just about the entire Copper family collection, for a start. "What's the Life of a Man?" and "The Drowned Lover" (both also in many other collections) are examples of songs which I found somewhat giggle inducing when I was late teens, but now at 60-odd I really like.

For contemporary songs Jez Lowe is one name I'd put forward, out of many.