Picking someone to entrust our daughter to...?

My husband and I want to "put in writing" so to speak who we would want our daughter to go live with if something were to happen to both of us. We want it to be legal because we have a feeling that there would be a little uproar if there is nothing saying our wishes. We want to know who would be taking care of our daughter. If anyone has done something like this, please tell me how you picked someone and what you did to make it official. Thank you!