Apu Nahasapeemapetilon.
pffffft. What does Paul McCartney know? Was HE the Fifth Beatle? I think not.
Bill Murray the K. Oh, no. Wait. Wrong band. 
It’s about time someone mentioned Murray the K.
Did he ever play “Rio” by Duran Duran?
Having enjoyed Back Beat, I gotta go with Stu.
I am afraid that I am going to have to make a similar post with Apu’s name in place of Apu’s name.
Billy Shears.
Most contribution is clearly George Martin, who had as much to do with their success as any of the Fab Four.
To my knowledge, various Beatles themselves named only four men as “fifth Beatles”: Brian Epstein, Billy Preston, Neil Aspinall, and Derek Taylor. Surprisingly, the folks here don’t seem to share the Beatles’ view about the last two; in our poll they’ve got only three votes between them.
And poor Jimmie Nicol—85 participants in the poll so far and no one has voted for him!