The Beatles' A Hard Day's Night (1964)
http://www.youtube.com/watch?v=csMJH1z6qNY
The first time the same single and album had ever held the number one spot in both Britain and America, 'A Hard Day's Night' is also the moment where The Beatles really began to emerge as a force to be taken seriously not just for their popularity but for their creativity.
It's opening chord still raises debate, with no absolute conclusion as to how it was achieved. The instrumental break also showed early signs of the band's interest in studio trickery, with George Martin's piano solo sped up to sound more like a harpsichord.
Either way, it's my Essential Song #4.