I listen to all sorts of shit. Sigur Ros (less than 1), Mogwai (1-5, depending on song), Boris (5-10), Ahab (10), Motograter (8-10), Isis (maybe about 3-11), Iwresteldabearonce (10-11), Neurosis (0-11), The Church (3, tops), and so on. I do listen to a love the fuck out of a lot of the really heavy stuff (especially the slower genres, like doom/funeral doom especially, and more melodic stuff) but my all-time favorites tend to be towards the lower end (Sigur Ros, Mogwai, This Will Destroy You, The Go-Betweens, etc).
And some better examples (in my opinion) for higher numbers of heaviness:
http://www.youtube.com/watch?v=xTVwqRJnNBY (Ahab, "The Sermon")(this is about a 12 for me)
http://www.youtube.com/watch?v=zoEY5NoaMuw (Rosetta, "Wake")
http://www.youtube.com/watch?v=rg2mEfush5A (Swallow the Sun, "Sleepless Swans")
http://www.youtube.com/watch?v=k7pbgggLTp0 (Rammstein, "Mein Teil")
http://www.youtube.com/watch?v=xEVyDjxsUrk (Neurosis, "Locust Star")(this one is easily a 10 for me)
The main riff (comprising of he majority of the song) of this is somewhere towards the upper end but I don't know where (warning: drone music ahead. Long live Sunn O))), the masters of drone): http://www.youtube.com/watch?v=CK5mN8ha5Xg (pt 1) http://www.youtube.com/watch?v=OF3MLCCY9z8 (pt 2)