There are a few different ways of measuring how damaging the cult is. It's definitely not as controlling as scientology from what I understand, so in that way I don't think it could be ranked a 10. Maybe a 7.5 or 8.
As far as cost in human life, I still don't think they're quite a 10. With the heaven's gate cult it appears that essentially all the members eventually died. With People's Temple (Jonestown) they had a membership at one time from somewhere between 3-5k people but a few less than 1000 died from the cult. While JWs definitely kill more and have a much greater absolute cost in human life, they aren't as bad on the cult scale because relative to their size they don't cause as much damage.
Now, where I a humanitarian or government entity looking at which cult to focus on in order to keep people safe - JWs would 100% be at the top of that list. Their size, absolute death toll and relatively benign public image (though that seems to be changing a little) make them the most important priority as far as shutting down dangerous cults goes.