I actually got into Metallica through their later (90s) material, particularly the two LOAD albums, so I tend to go back and forth on what I find to be their best work/era.
This is most likely their definitive musical statement though. The band had been steadily developing and refining their sound over the preceding two albums. This album follows the pattern of most of their 80s albums: first track has a mellow intro that crashes into a speedy thrash riff, the second track is a monster title-track, track 4 is the ballad/mellow song, and the last two tracks are an instrumental and a hard-n-fast thrash track.
The band is firing on all cylinders for this album, with all 4 members contributing to the songwriting and arrangement, and all taking time to solo or showcase their chops. Even the oft-mocked drumming of Lars Ulrich is tight and effective here.
The songs move between straight-ahead, fast thrash, to more complex, densely harmonic (almost classical) arrangements in songs like the title track, and the instrumental "Orion", this latter largely composed by Cliff Burton and featuring some stellar bass soloing from him.
Burton died in a bus accident while touring for this album. The band would seek a more uniform sound in their follow-up "...and Justice For All". They would simplify their sound for their 90s albums, then meander for a while before finding their way back to attempt to recreate their 80s masterpieces... but with mixed results. While their 90s albums were successful and enjoyable, they don't reach the same heights of collaboration, and ambition as Master of Puppets.
