No. 1: The Beatles - Sgt. Pepper's lonely Hearts Club Band (1967)
For me, this is ground zero - where what we accept as classic rock comes together. For many, this was the first time an album brought together all the elements that make up an album.
There was a lot to digest when this first came out - certainly more than just the music. At a time when many albums were vehicles to collect past hit singles plus some filler tracks, Sgt. Pepper was meant to be an album where there was a flow and a narrative from song to song. The idea came to be known as a concept album and many bands attempted it. But, arguably, The Beatles set the template. It was an album of new music. There was a story that connected the songs. The band itself played the parts of a fictional band. The bookending of the album, more or less, with an opening and reprise.
The packaging was as innovative as the music inside. A gatefold cover, the cutouts insert, lyrics printed on the cover, and that wonderful front cover of the band amid celebrities, cutouts, and some in jokes. There was a lot to look at while you listened to the album.
You could probably spend a day or more reading about the music or the impact the album had at the time. And you could certainly spend a week or more listening to the album and it's various reissues and remasterings. Then there are the tribute albums inspired by Sgt. Pepper.
All of which are tributes to the importance and influence of Sgt. Pepper's Lonely Hearts Club Band in popular culture. And why you should have it in your collection.