Cassidy.
Well, okay, sure, there is slightly more to it.
The obvious reason for the popularity of "Who is Golden Freddy" theories is that there should be something special about them.
They are the spooky secret 5th animatronic who teleports and produces hallucinations and breaks corporeality in odd ways.
They are the original secret character, before FNaF2 carelessly threw a bunch at us.
They are clearly special somehow.
Yet, their basic origin story doesn't really justify this.
Golden Freddy is Cassidy, who died alongside the others, without any real unique circumstance as far as we know.
You can throw out ideas like "she was springlocked" or "she wasn't stuffed" or whatever, but it doesn't do much to justify the narrative weight the character is given.
There is a mismatch between the special 'mysteriousness' to Golden Freddy's portrayal, and their apparently unremarkable origin as just another of the five missing kids.
"What's the deal with Golden Freddy" really should be one of the story's central mysteries, shouldn't it?
Well, I don't think it's quite as bad as it seems.
I have of course neglected to mention that Golden Freddy is Fredbear, which arguably can justify their narrative weight.
Fredbear is "Where it all started", the origin of the Freddy's brand, and the compliment to Spring Bonnie, who is tied to Afton.
Golden Freddy is often treated as a 'leader' for the missing children, or as a representative of the group.
Thus, the original two animatronics represent the children and Afton, the central duality between the main characters of the story as a whole.
That might justify Golden Freddy's relevance in one sense, but it doesn't really explain why they're all spooky and whatnot.
The popular explanation, I think, is GoldenDuo- Golden Freddy is especially spooky because he has two spirits rather than one.
This is inspired by the Stitchwraith in Frights, who has three spirits, and notably does not show any of the 'spooky' behaviors Golden Freddy does.
Not that it matters, because ShatterVictim kills this idea regardless.
BV is in all the Classics, not just Golden Freddy.
BV can still save the day, though, through MemoryVictim.
Even if BV's spirit isn't exclusively in Golden Freddy, he nonetheless has a special connection to him which is highly emphasized.
Fredbear is the character BV feared most, as seen in the nightmares, and he's the character who ultimately kills him.
The special spark where all the suffering of Freddy's originates, the bite, revolves around Fredbear, and is initiated physically inside of his head.
BV's fears, the fears which haunt the story as a whole, are presumably channelled most directly into Fredbear himself.
Following this, the mystery of Golden Freddy's nature is equivalent to the mystery of FNaF4, which is ultimately the big mystery of the story as a whole.
It works out.
(Yes I know BV doesn't exist in the novels.
I don't think it matters- they aren't a "what if" scenario, they're a different take on the story, with different priorities.)
Could this have been done better if it was planned from the beginning?
Almost certainly.
Fredbear's importance here has very little to do with Cassidy, which is a bit awkward, arguably.
By all means, consider ways to give Golden Freddy special importance when developing AUs.
I think that's all I have to say about that.