Sgt. Pepper's album

Sgt. Peppers represented a generation both musically and culturally. It distilled the spirit of 1967 and the “Summer of Love”. It was not only a cultural milestone, but also a decisive moment in history of western civilization. It is even considered to be a microcosmic history of pop music and of the 20th century. 
That being said, it also brought about a clear demarcation point among the younger generation and that of the older, particularly in regards to musical taste.
Not to mention, the many layers of studio tricks found on the album ushered in the headphone era, isolating the listener from the rest of the world around him or her.