What's an album where people claim that nothing was the same after it was released, an album that not only shook up the music world going forward but that hugely impacted pop culture as well?
I'm going to go with The Beatles Sgt. Peppers Lonely Hearts Club Band. This was the album that proved to the world that pop/rock music could also truly be high art, it was touchstone in the development of the concept album, and it captured perfectly the mysticism and optimism and non-conformity of youth culture of the time.
Someone else already mentioned the album in the comments, but I'll say it again - Black Sabbath's debut album Black Sabbath.
It wasn't even that popular when it was first released in 1970, but it was a very impressive beginning for the band credited with creating an entirely new genre (heavy metal). Not only was it sonically heavy (there were bands at the time that were arguably just as heavy), but it also had an atmosphere of darkness and lyrically tapped into that, in a way that the other big bands of the time didn't seem to do.
As the decades pass, metal would go on to be way heavier and some would of course consider Black Sabbath as "light" or just plain rock. Yet to this day, many bands and metalheads who know their history still pay their respects to these guys.