Lady Olenna Tyrell
Why it could be her:
- In what may have been foreshadowing, Lady Tyrell approaches Sansa during the reception to say how terrible it is that Robb and Lady Stark were killed at a wedding, commenting on how awful a person would have to be to murder someone during a special occasion.
- Lady Tyrell expresses her concerns about Joffrey several times, asking Sansa for her opinion and worrying about her granddaughter marrying such a tyrant. Killing Joffrey would get Margaery out of harm's way.
Why it couldn't be her:
- Evil or not, King Joffrey was her granddaughter's new husband, and she rushes to Margaery's side to comfort her after he dies.
- Lady Tyrell's conversation with Margaery implies that she's too smart to have killed Joffrey — at that point anyway. She explains to Margaery that it would have been better had they consummated the marriage first, this guaranteeing her right to the title.