@Quaternion.FromToRotationを使用する方法1
 transform.rotation = Quaternion.FromToRotation (Vector3.right, direction);

AQuaternion.FromToRotationを使用する方法2
 transform.rotation = transform.rotation * Quaternion.FromToRotation (transform.right, direction);

BVector3.Angleを使用する方法1
 transform.rotation = Quaternion.Euler(0, 0, Mathf.Sign(Vector3.Dot(Vector3.forward, Vector3.Cross(Vector3.right, direction))) *Vector3.Angle(Vector3.right, direction));

CVector3.Angleを使用する方法2
 transform.rotation = transform.rotation * Quaternion.Euler(0, 0, Mathf.Sign(Vector3.Dot(transform.forward, Vector3.Cross(transform.right, direction))) *Vector3.Angle(transform.right, direction));