The first 2 albums may be seminal but quality-wise, Master of Reality is the band's first true classic. Whereas Black Sabbath was a heavy blues album and Paranoid was a psychedelic blues album, Master of Reality is the first real statement from the band that earned their coined 'heavy metal' tag.
From the very onset, it's all chugging, down-tuned, fuzzy, big sounding riffs that are synonymous with what heavy metal is today. It is also really their only true heavy metal album, strangely as blues and rock and roll pervades most of their Ozzy catalogue.
