Who was the fifth Beatle?

That is open to debate. Stuart Sutcliffe was a fifth member early on, producer George Martin was a fifth musician with them in the studio, and they worked with occasional guest performers. Manager Brian Epstein was called a "fifth Beatle" by the band themselves.

A New York City radio DJ called "Murray the K" (Murray Kaufman) declared himself the "Fifth Beatle" when the band first visited New York. He had a popular show, and gave the Beatles endless promotion on that show, which they appreciated, so they let the nickname stick.

Many people have been referred to as the "Fifth Beatle" at different times, but the most obvious and probably top of the list being their long time recording guru and producer, Sir George Martin, another being long time Roadie, Assistant and CEO of Apple, Neil Aspinall.

Oodles Answer: Phil Spector