When I watched I thought it was this person for sure and I have no information outside of watching the show. If I'm wrong, I'm wrong.
Who was the person with the greatest revenge reason to poison Joffrey?
When was the only time the King's cup was not publically visible to the others?
Who had access to the cup at that time?
Why did this person need to hold their right hand over the top of the cup other than to drop something into it?
Who wanted to suddenly wanted to leave the party shortly after the King put his lips to the cup for a little sip?