I really can't choose! Zoë had a terrible past, as did Bianca. Both of them died (don't mind me crying, please...) on the same quest. And Zoë knew she was going to die, while Bianca even thought she had heard of the prophesy, still went. I love them equally.
I liked Zöe better, but I think I would rather have Bianca not die because of Nico.