This is a term that is thrown around by a lot of people to refer to someone who greatly helped
The Beatles to make such
great music. People point to a wide variety of people as being this "fifth
Beatle," but it really would be impossible to say.
Some names that are commonly associated with this title:
George Martin
The Beatles's producer, he was responsible for much of the arrangement that occurs on their albums. He usually arranged the orchestral overdubs, such as those in A Day in the Life (Though he wasn't beind Let it Be). He is my pick.
Brian Epstein
The Beatles's manager, he helped bring them into the limelite, and even devised the idea of putting the boys in suits. He was a major impetus behind them getting a record deal, and was rarely seen away from them while he was alive. He was definitely important in bringing them to the public eye. Of course, he also probably cost them millions of dollars worth of merchandise revenue and let to much of their financial mess.
Pete Best
I guess that tecnically, Pete Best was the fifth Beatle, removed from the band in favor of the more talented Ringo Starr.
Stuart Sutcliffe
The original bassist for the band, he played with them during most of their dates in Germany. He wasn't very good, and was in the band only because he was friends with John and Paul. He often played with his back to the audience so they couldn't see that he wasn't very good. His girlfriend, Astrid Kirchherr, took many of the early photos of the band, including the most famous one. Stuart died of a brain hemhorrage in the early 1960s.
Neil Aspinall
Neil Aspinall was The Beatles' roadie. He was with the band for nearly all of their career, starting a while before they were signed. He continued to help the band as much as possible, eventually taking a job at Apple Records. He also contributed ideas and backing vocals to a wide variety of songs.
Billy Preston
When people start to bring up Billy Preston as a possible name for the fifth Beatle, it starts to get iffy. He was talented, and he played piano and organ with the band on most of the songs on Let it Be (from the Get Back sessions). He also appeared at The Concert for Bangladesh with George Harrison, Ravi Shankar, Eric Clapton, and Bob Dylan. But, he wasn't with them at all before that, and calling him the fifth Beatle is quite a stretch.
Murray the K
Murray the K was a radio DJ in New York City in 1964 when The Beatles first arrived in America. He was, at least partially, responsible for the incredible popularity of The Beatles there, and some say that he was responsible for the huge turnout at JFK Airport when The Beatles landed. It is from him that the term originates. He somehow managed to make his way into the band's suite at The Plaza Hotel in New York City, and he began broadcasting, declaring himself to be "the fifth Beatle." Thus, the phrase was born. While he did help the band to become popular, he didn't do quite enough to achieve true "fifth Beatle" status.