Each Beatles record was an advance and a revelation of some sort.  Each album introduced something creative and new. Sgt. Pepper, however, rose above the rest.  It represented a quantum leap beyond what had gone before, beginning with the cover.  It was an explosion of imagery and artistry and innovation from beginning to end.  (That might not be apparent now, over 40 years later, as the innovative techniques which the Beatles inspired and pioneered have become commonplace.)  The lyrics, the tunes, the voices, the harmonies, the instruments, the orchestrations and the technical wizardry of George Martin all conspired to reflect the spirit of the time.  This might be best embodied in the aural perfection of the closing "A Day in the Life."  The album continues to amaze me.  When you buy Sgt. Pepper's Lonely Hearts Club Band, as you should, turn down the lights, turn up the sound, and just listen.  It's a trip!