Seems to me it makes the most sense it was the queen of thorns who did the deed. Sansa told them that Joff is a vile person and their overall impression was "Well that sucks but alright." Which I'm taking as "Well if we can't have a happy marriage we'll kill the fucker." 9/10 in fiction a death by poison is from a female's hand so it fits in with that. Queen of Thorns seems to be the only female in the entire city both smart and capable enough to pull it off. All the other females can't stop themselves from fucking up all the time. It may also account for why she was so salty about having to pay so much cash for the wedding since she knew it was just a charade and excuse to kill the king.
I still think Joff was a great character. But I think his act would have just worn on had he not been killed off. He wasn't deep enough as a character to keep around.
I still think Joff was a great character. But I think his act would have just worn on had he not been killed off. He wasn't deep enough as a character to keep around.