Sgt. Pepper's Lonely Hearts Club Band is, in my opinion, the most historically important album ever released.  This album revolutionized the rock and roll industry; often imitated but never duplicated, it continues to exert a significant influence on the very business of singing and recording music.  Even the timing of the album was perfect; the year was 1967, the Summer of Love, and this album captured the spirit of the times in such an amazing fashion that other recording artists immediately recognized the profound significance of what the Beatles had just succeeded in doing.  I remember seeing an interview with David Crosby, who basically said that Sgt. Pepper accomplished what he and so many other artists of that war-torn generation had been trying to achieve themselves - musically, artistically, and culturally.