Paranoid is the best and arguably the first metal album is unrelenting in its darkness and awesomeness. An incredible album from beginning to end, there is no question of its influence in changing and molding the rock scene to something much heavier. I doubt nearly anyone or anything on this list would exist, at least not in the same way without Black Sabbath.
The music is not terribly complex but quite catchy and the heavy lyrics are charmingly awkward at points, but rather than being a distraction, any quirks in the production just elevate the music. In short this is pretty powerful stuff, truly an album that changed the world.
And I just have to mention the magically strange and slightly off cover art which is more now than now and one of my favorites of all time. Here, here, here, and here are some videos for your enjoyment.