George Martin, of all those who've been bestowed the honour of 'Fifth Beatle', had by far the strongest claim.
Over the course of the 1960s, his piano skills, background in comedy and classical music and technical expertise helped the band become the legends they are.
Tributes have flowed in from his former protégés since his death yesterday at the age of 90. The news was broken by Beatles drummer Ringo Starr on Twitter.
Martin's influence on the Beatles' sound cannot be overstated. Here's the true Fifth Beatle's finest moments with the band.
Inspired by Bing Crosby and Roy Orbison, John Lennon's harmonica-drenched hit began life as a ballad.
"At that stage 'Please Please Me' was a very dreary song," Martin once explained. "It was like a Roy Orbison number, very slow, bluesy vocals. It was obvious to me that it badly needed pepping up."
Eighteen takes later, Martin told the band they'd just recorded their first number one hit -- and he was right.
Saying Martin fired Pete Best from the group might be overstating it a little, but it was his call not to use the quiff-wearing drummer on their first proper studio recordings.
"It was pretty tough for him and I felt guilty because I felt maybe, I was the catalyst that had changed his life -- so I'm sorry about that, Pete."
But Pete wasn't the only one not to meet Martin's exacting standards -- Ringo's first day in the studio as a Beatle saw him relegated to tambourine, while a session muso banged the drums on 'Love Me Do'.
By late 1966, Lennon's mind had been opened (or addled, it's up to you) by LSD, and his compositions were becoming increasingly strange -- few more so than 'Strawberry Fields Forever'.
Happy with the first half of one take and the second half of another, Lennon tasked Martin with finding a way to piece them together -- despite the first half being recorded at a different key and tempo to the first.
Martin, with help from engineer Geoff Emerick, managed to turn the mess into one of the Beatles' greatest tracks -- using a four-track tape machine, no digital.
Paul McCartney wanted to bridge the gap between his and Lennon's sections of 'A Day in the Life' with a full orchestra making a hell of a racket.
Martin, having a background in classical music, knew the musicians wouldn't be able to properly improvise the atonal crescendo McCartney had in mind -- so he wrote a score for it.
"What I did there was to write... the lowest possible note for each of the instruments in the orchestra," Martin explained.
"At the end of the 24 bars, I wrote the highest note... near a chord of E major. Then I put a squiggly line right through the 24 bars, with reference points to tell them roughly what note they should have reached during each bar."
Sure, you or I could have done that -- but Martin was the genius who knew it would work.
Of course, none of this would have happened if Martin hadn't signed the band in the first place.
He hadn't seen them live, called their demo tape "unpromising" and thought their original songs weren't very good. So why did he sign them?
After asking the band if there was anything they didn't like, and George Harrison -- then only 18 -- told him, "Well, there's your tie, for a start."
This is where Martin's background in producing comedy records came to the fore, deciding to sign the future biggest band in the world not because they were any good -- but because he thought they were funny.
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