If I were to make a personal top 25 metal albums list, it would include Slayer but (now for the controversial part) it wouldn't include any Black Sabbath. Don't get me wrong. I like Sabbath, but I just think there are much better bands out their in terms of musicianship and songwriting skills.
I'm talking metal AND non-metal in my list, which is why Slayer doesn't make the top 25