Yeah I like Melissa Auf Der Maur. Also Kim Deal (Pixies/The Breeders), she hasn't got a huge voice like some, but it's really nice.
But if I had to choose one, it would have to be Lillo. I could listen to her sing all day, and i'm sure Lacuna Coil would be a completely average band without her. Close second is Otep, she's great too.