Their most influential album It could be said that Kill 'Em All, the band's debut album, was responsible for popularising Thrash metal, but it was Ride The Lightning, its follow-up, that created the definitive version of Metallica. This album expanded the band's musical horizons and broadened their palette, without losing any of the power of their first album. 

  

If in Kill 'Em All Dave Mustaine's shadow was still too long and his weight was more noticeable than that of the two members with less compositional skill, Cliff Burton and Kirk Hammett, here, however, you can enjoy ‘the four-headed monster; James Hetfield and Lars Ulrich are still the main composers but the excellence as musicians of the other two is also strongly noticeable in all the pieces.   

Burton's musical knowledge is fundamental to this album in which he taught his bandmates the principles of music theory, something that is noticeable in the greater harmonic and melodic complexity of the songs on the album. This can be seen from the outset with the acoustic opening Fight Fire With Fire, in which they sound like medieval minstrels, which makes the appearance of the overwhelming opening riff, with Hetfield's '84 Gibson Explorer 'So What', all the more explosive. 

  

Though perhaps the defining moment of the album, and of Metallica's entire career, comes with Fade To Black, the moment when the most important band in the history of Thrash delivers their first ballad, causing, for the first but not the last time, many of their fans to tear their hair out and accuse them of selling out. The fact is that the formula of this song is one that most of their ballads would follow, on monuments like Welcome Home (Sanitarium) or One, with Hetfield demonstrating that he is a great singer on the quiet beginning, and then an explosive second part in which the heavy riffs go in with a knife. By the end the band has transformed itself into the 2.0 version of Black Sabbath for an anthological climax, Kirk Hammett's solo included. 

That greater musicality I was talking about is also noticeable in the title track where we see that you can sound brutal as well as sophisticated, thanks to Cliff Burton's incredible work on the four strings. Although this is one of the two songs that still features Mustaine as co-writer, one of the riffs is his; you can see in Kirk Hammett's Flying V solo that Hammett has stepped out of his shadow to start flying free. Another one of the album's fundamental monuments is For Whom The Bells Toll, a song that begins with ominous and menacing bells, as if they were the same bells of hell that AC/DC rang out. The song builds slowly, riff by riff, until Hetfield's voice enters after two minutes, and then the storm is unleashed. 

  

To round it all off, the second side closed with two more huge songs, first came Creeping Death, an explosive song composed from the point of view of the Angel of Death, in which Metallica sweeps away everything in its path. Finally, closing the album was the instrumental The Call Of Ktulu, almost nine minutes of brutal expertise that also came from Mustaine's time in the band.   

Ride The Lightning was the album with which they found their magic formula, a formula that they continued, even improving on it, with Master Of Puppets. That's why it is also their most influential album, and there wasn't a single metal band, but also hardcore and punk, that didn't fall under their enormous influence, which made them the most important band of the genre.