Maxwell's Silver Hammer

Entry from Wikipedia:

"The song took three days of overdubbing because McCartney imagined that it could be a future single. John Lennon later recalled, "he did everything to make it into a single, and it never was and it never could have been."
According to Lennon, the band spent more money on that song than any other on Abbey Road, and he derided the song at the time as a prime example of McCartney's "granny-style" writing.
George Harrison described it in 1969 as "one of those instant whistle-along tunes which some people hate, and other people really like. It's a fun song, but it's kind of a drag because Maxwell keeps on destroying everyone like his girlfriend then the school teacher, and then, finally, the judge." In 1977, Harrison would be less charitable, stating "I mean, my God, 'Maxwell's Silver Hammer' was so fruity."
Even Ringo Starr recalled in an interview in early 2008: "The worst session ever was 'Maxwell's Silver Hammer.' It was the worst track we ever had to record. It went on for fucking weeks. I thought it was mad."