Cersei, as queen. Jaime, as a member of the King's guard, and Tyrion, as Hand of the King (and over various roles over time). Yes, Joffrey openly defied Tywin on several occasions, but Tywin is the kind of person that plays the long game and will use a person's interests to further his own interests.
Joffrey takes multiple chugs of wine that is supposedly so poisoned it has turned purple, then shows no reaction while Margy says an even longer sentence about Lord Buckler, Joffrey taunts Tyrion about his pie, reaches down, taunts again, shoves pie into his gob, taunts again, grabs another piece, bit dry, needs washing down, drinks wine -- and
bYcM6kA.