It's a fun question to consider in a figurative sense...all in the list can be "5th" in a figurative sense but, it's fun to look at that as possibly being a trick question. Consider the chronology.
Look at the band's timeline, and you'll quickly see was the original 5th beatle in the literal sense of the term: no one.
here's why: An existing foursome was called the "silver beatles", and soon added a 5th silver beatle as drummer.
Soonafter, with "silver" being dropped, all 5 "silver beatles" simultaneously became an original beatle. It's not as if there were 3 "beatles"..then a 4th..then 5th, then back to 4. In reality, PB was the 5th Silver Beatle, but then at time of name change in 8/60 all 5 instantaneously became "a beatle", so no one was 1st, 2nd, 3rd, but in the history of the Silver Beatles, PB was the original 5th silver beatle. Stu was never a 5th in the literal sense, but became a figurative 5th when he quit the band. It's also documented by the consummate authority (Neil Aspinall) that either Pete or Stu would have to be considered the "5th". There were 6 total people on the planet that were "a beatle". Thankfully, 3 of them are still with us. Fun question up top, but why wouldn't pb even be on the list ? (for fun sake, could even add chas newby as didn't he sit in on bass as a beatle for the first "beatles" (not quarrymen) shows at casbah after 1st hamburg trip?.
All in good fun though. thanks for the survey question..that was just my 2 cents for the sake of fun tidbits...see ya