There are two primary ways to ensure an album is immortalized as being truly essential: a) the perfection of an existing style or idea. b) The creation of entirely new idea. Of course, there are several other factors at work, lyrics, musicality, general ability, and so forth, but should the artist be able to fulfill one of these two requirements, history will be far more forgiving for any other shortcomings on the album's behalf. These two factors are what can separate a good album from a truly great one.
So, with that in mind, I'd like to present two albums, that, born from similar circumstances succeeded at ensuring immortality via the two methods mentioned above: The Basement Tapes by Bob Dylan (and the Band), and Trout Mask Replica by Captain Beefheart. The Basement Tapes succeed so masterfully at capturing the idea of America in music that they feel as though they've been engraved as a part of America's cultural history. They feel at once aged yet timeless, and rest so naturally inside some great, mythic America that act as almost a cultural road map to Americana. Trout Mask Replica, on the other hand, creates not only a new idea, but an entirely new America, the songs act like a Basement Tapes for a world that never existed, the songs are like folk songs for a folk that never were, the words fitting into some colloquial that we will never really hear, and will never understand. Together, these albums have reached a more or less immortal status (if not in the public conscience, then at least in the heart of critics), and stand as testaments to the ideas of perfection and innovation in music.
Both albums sprung from similar backgrounds. Both arose from the brain of an eccentric songwriter, who'd been secluded from the world, working and living with his band for an extended period of time. Both albums were recorded towards the tail end of the 60s (although The Basement Tapes wouldn't see official release until the 70s), and both were completely, and entirely unlike anything else being released at the time.
There were differences, of course, The Basement Tapes come from a far more relaxed environment, more or less the loose recordings of a seemingly endless jam session, while Trout Mask Replica was the product of intensive rehearsals, driven by the perfectionist Beefheart to replicate the noises in his head, fighting against the limitations of technology at the time. And of course, the end results were wildly different, where Dylan's songs sounded at once familiar, even to those who'd never heard them, Beefheart's were wild, thrashing affairs that sounded unrecognizable, even on repeated listenings. In fact, only one of Beefheart's songs (the warbling blues of "China Pig") was even in a genre that could be recognized. Meanwhile, Dylan's songs seemed to epitomize folk even more than anything he had done before, this truly was the music of the people, whether the people really existed or not was irrelevant, it was the music of the American people, the ones from legends and traditional tales, and the history therein.
The successes of these albums lie in how effectively they manage to fulfill the tenets mentioned at the beginning of this piece. Both albums have their weaker moments ("Bessie Smith", "The Blimp"), but because of their mastery of these aforementioned concepts, these failings are forgiven, embraced even, as a testament to the sprawling reach that led to such monuments of artistic creation. And it's monuments such as these that dot the landscape of music, and it's monuments like these that form the peaks of music history.