It's their strongest album.
Master of Reality is their tightest and most consistent effort but Sabbath isn't at their best at their tightest and most consistent. I'd hold up the two best on Master of Reality as After Forever and Lord of this World. Into the Void also justifiably get's held up for high praise and Sweet Leaf, while a tad overplayed is excellent. Then of course their is the excellent and criminally underrated Solitude. However, that is balanced somewhat but the criminally overrated (but still good) Children of the Grave. Then there are the little acoustic interludes but that's about it for the album. The album has the least musical range of any Ozzy era album.
I'll compare it to Paranoid as its most similar work (Black Sabbath is my favorite of theirs but it is such a raw musical statement that it really stands by itself). Both have the crushingly heavy track (Into the Void and Electric Funeral). Both have the catchier slow riff number (Sweet Leaf and Iron Man). Both have an achingly beautiful soft number (Planet Caravan and Solitude). Both have their faster paced poppier numbers (Paranoid and Children of the Grave). But let's compare the two centerpieces long songs from the album. In all cases except the first Paranoid outclasses Master of Reality. For Master of Reality it is the two I mentioned earlier. For Paranoid it is War Pigs and Hand of Doom. Master of Reality's are great but they are both very similar. War Pigs and Hand of Doom are both quite different (indeed Hand of Doom is like few metal or even pop songs as it climaxes about a halfway through and then slowly diffuses tension without leaving much resolution and starting as quietly as it began). War Pigs builds more like a traditional epic but it is Sabbath at their songwriting peak. I haven't even mentioned Fairies Wear Boots.
Of course Paranoid has Rat Salad which is a bit unnecessary and weaker than anything on Master of Reality. And it's pacing and structure probably isn't as tight and its theme not as consistent. It feels in some senses more like a collection of songs than Master of Reality but that's what really works about it. It is Sabbath exploring tons of musical ground and expanding on their concept. It doesn't have the raw unbridled power of the first which is why it falls short but it is what they do best.