The Pink Letter
Theon is most likely the author of the Pink Letter. Others have already made videos regarding for and against arguments for others, but to my knowledge no one has yet suggested Theon. And yet he is the best possible suspect. I won’t go over the positive or negatives of any other suspect. This is strictly about Theon.
He has the run of Winterfell, even showing Lady Dustin the crypts.
One can safely assume he has access to the rookery.
He knows Ramsey better than anyone. Better even than Roose, I suspect. Nobody would know better than Theon how he talks, or his obsession with being called a bastard.
He also knows how much Jon hates being called a bastard.
He met and talked to Mance and his spearwives
It’s true we don’t know the full extent of their conversation, but we do have hints that Mance told him some things. And he doesn’t know that Jon sent Mance’s son away.
He would also have heard about Melisandre while also being ignorant of her actual location.
Theon would not have been able to obtain any skin, so he couldn’t have copied Ramsey on this point.
He wouldn’t have had access to the Bolton seal either.
He, of course, knows better than anyone that Ramsey refers to him as ‘Reek’.
And here is the final point, Theon has lost several fingers. This would surely screw with his writing. Thus, his writing might easily have been mistaken for Ramsey’s, who has atrocious handwriting. (The writing used in the background picture is far too legible.)
It has long been speculated that Theon is suffering from a split personality. One could easily see him writing the Pink Letter during a disassociated episode.
Theon also has the strongest motive of all for wanting Jon to come south, to save him. Certainly better than any suggested for Mance. Nor would he be sorry if Jon died in the process.