I'd say their best songwriting (Lennon/McCartney) was during the three year from 1965-1967. After that, most of their songwriting was John or Paul writing the song individually, and both were getting a writing credit on the song. "Across the Universe," "Don't Let Me Down," "Julia," and "The Continuing Story of Bungalow Bill" were clearly Lennon compositions. While "Oh Darling," "Let It Be," "I Got A Feeling," and "The Long and Winding Road" were clearly McCartney compositions.
Their best session work was during
The Beatles - White sessions... most of the songs that didn't make it there on the
White album found their way onto the
Abbey Road album...
Their best album is
Abbey Road ... personally because I'm biased towards George Harrison's "Here Comes the Sun" and "Something".