With Paranoid, Black Sabbath perfected the formula for their lumbering heavy metal. On its follow-up, Master of Reality, the group merely repeated the formula, setting the stage for a career of recycling the same sounds and riffs. But on Master of Reality Sabbath still were fresh and had a seemingly endless supply of crushingly heavy riffs to bludgeon their audiences into sweet, willing oblivion. If the album is a showcase for anyone, it is Tony Iommi, who keeps the album afloat with a series of slow, loud riffs, the best of which -- "Sweet Leaf" and "Children of the Grave" among them -- rank among his finest playing. Taken in tandem with the more consistent Paranoid, Master of Reality forms the core of Sabbath's canon. There are a few stray necessary tracks scattered throughout the group's other early-'70s albums, but Master of Reality is the last time they delivered a consistent album and its influence can be heard throughout the generations of heavy metal bands that followed.