getClosestFace.cs

    //return closest face to user
    public Transform getClosestFace(Transform[] child, Camera cam)
    {
        Transform closestFace;
        try
        {
            closestFace = child[1];

        }
        catch (Exception)
        {
            closestFace = child[0];
        }
        foreach (Transform face in child)
        {
            if (face.childCount != 0)
            {
                continue;
            }

            Plane[] planes = GeometryUtility.CalculateFrustumPlanes(cam);
            float closestFaceDistance = GetVisiblePercentage(closestFace.GetComponent<MeshRenderer>().bounds, cam);
            float faceDistance = GetVisiblePercentage(face.GetComponent<MeshRenderer>().bounds, cam);
            if (faceDistance < closestFaceDistance)
            {
                closestFace = face;
            }

        }
        return closestFace;
    }