For me it’s probably a tie between my best friend from college, my baby sister, and my former therapist.
Currently Sis is slightly ahead. I lent one of our nieces my debit card earlier so she could do some shopping for a family dinner, and when my niece asked for the PIN, my sister told her it was probably one of 4 sequences: the first four digits of Avogadro’s number, pi, Planck’s constant, or e. She was right. I’ve never told her how I choose my PINs; she just knows that’s what I think is easy to remember.
But I don’t doubt my college friend could have made the same deduction. She can also predict, almost infallibly, what I will order if we are visiting a restaurant new to both of us, or what movie I’ll choose to see, or whether I’ll like a given author.
(Athena help me if the two of them decide to join forces and whack me. I’d never escape.)
Of course, my therapist knows things about me that neither of them do, as I do not discuss sex with my little sister.
My husband knows me best, hands down. And thank goodness, because I couldn’t have picked any of the other options - grandparents deceased, no brothers or sisters, no best friend, and no therapist.
Also none of the above. My Aunt, to whom I have always been incredibly close, knows me better than anyone in the world. She knew me when I was a tiny baby, a precocious kid, a religious fundamentalist teenager, and an adult coming into her own. We bonded like sisters. We’ve been through everything together, and she is the one other person who knows what it is like to be sane and healthy and related to the rest of our family.
He wouldn’t be able to guess my PIN, because it’s based on an obscure nickname someone gave me in high school–he’d know it was something like that, but of course he would have to be told just which nickname and how it’s been changed. I can tell you what he’ll base his PIN on in the same way.
My husband knows me extremely well - he can predict me almost exactly. Oddly enough, I don’t know him very well at all. Of course I know the basic person that he is but there are many many situations in which his reactions and opinions are a complete surprise to me.