Okay, I will offer my theory of who is the first Beatle as follows:
1) Paul 2) George 3) John 4) Ringo
Here is my reason:
Paul already knew George before he met John. Therefore, there were already two Beatles who knew each other and played together. Then Paul met John and eventually acquainted George with John. Paul was the causal link for the triune Beatles. Thus you now had 3 Beatles. And, of course, lastly Ringo.
Paul was also the last Beatle because he made the official announcement of the split. That's what counted; not John's informal declaration earlier.
I rest my case because it's all I've got anyway!