I wouldn't say Let it be isnt great..........it just pales in comparison to Abbey Road and the White Album. It has some indispensable songs (Across the Universe, Long and Winding Road, and Let it Be)..............but I also think its incorrect to say that Rubber Soul and revolver was their creative high point. It certainly was their turning point, but as song writers they certainly surpassed both albums. It seems to me that the Beatles, like most artists followed a particular path: and the pattern......they started off rather primitive and simple (love me do, i want to hold your hand, a hard days night)......these are still great songs of course, but they are downright simple compared to what came later.........the next phase was complex and experimental composition..........Rubber Soul, Revolver, of course Sgt. Pepper, and Magical Mystery Tour fit this description. But after this tenure, they began to write more emotional and contemplative songs. There style began to fuse the complexity of Revolver and Pepper with the primitive production of earlier albums like Beatles For Sale.......This is the White Album and Let it Be (which was recorded before Abbey Road)..........Abbey Road is not my favorite Beatle album (The White Album is), but Abbey Road is their crowning acheivement as a band I think. It has the complex writing of Pepper (maybe even more complex - Because, You never give me your money and the whole side two medley).....it has the primitive rock style of earlier times - Come Together, Oh Darling, it has the emotion of the white album thanks to Harrison....something and Here Comes the Sun, and as an album it flows better than any other Beatle album.
Rubber Soul and Revolver are where the Beatles began to change popular music from commercial to artistic. But they went even further I think.