Just going with the gut feeling I got from the shows set up:
It seems that show tries to make us think that it was either the Fool, Sansa or the guy with a kickass mustache. But it is more likely to be Cersei, Jamie or Tywin. Each of the three have motive and would benefit from the situation. Most suspicious is Tywin, since he is the hand of the king and would have the power to rule while Cersei's other kids grow up. Joffrey was too unpredictable for his grand schemes.
But that is just my guess, it could have just been very effective bird-flue from the unsanitary catering conditions.