Well besides Chandelier and Big Girls Cry, my single choices would be Free The Animal, Dressed in Black and Fire Meet Gasoline. But she could release Elastic Heart if she wants to. It can still be big, see Latch?
I don't like the solo Elastic Heart. The Weeknds voice spills the sparsity of the instrumental in that verse in a way Sias doesn't seem to be able to do as well.