Ramsay Snow/Bolton
The demands that the letter lays out mostly fit Ramsay’s character. He would understandably want Reek and Jeyne back, as they are like pets to him, and they would likely be seeking shelter with Jon. Wanting to kill Melisandre and the rest of Stannis’ family would also make sense, since he’d want to eradicate whatever remains of Stannis’ claim to the Iron Throne, which is what drove him to attack Winterfell in the first place. It also makes sense for Ramsay to constantly mock Jon for being a bastard, because even though Ramsay was legitimized, he is still insecure about being a bastard himself.
However, there are two main reasons why Ramsay probably did not write the pink letter. The first is that the letter’s style does not match other letters written by Ramsay. We have two examples to compare. Elsewhere in A Dance With Dragons, Ramsay sends a letter to Asha Greyjoy and one to Jon, before the pink letter is sent. And both of those letters are styled and described in the exact same way. Both are written in brown ink that is likely blood, with spiky handwriting and signed by Ramsay and the lords in Winterfell: Barbrey Dustin, Lady Cerwyn, the Ryswells and an Umber. They are also tightly rolled and neatly sealed with hard pink wax, and a piece of skin attached to the letter (because Ramsay is a sick cretin).
But the pink letter is different. The hard pink wax is smeared, and the other lords did not sign it, only Ramsay. There is also no mention of the spiky handwriting, the blood brown ink or the attached skin. The other main flaw is that Ramsay threatens to cut out Jon’s heart, even though his favorite form of execution is flaying.
So while the aggressive tone and the list of demands fit the bill, there are a few arguments against Ramsay being the author. It seems like someone had a general idea of how Ramsay would write a letter, but did not have enough knowledge or time to mimic his exact style.