Holy fuck. I missed the episode last night and just watched it now.
I think that was my most enjoyable viewing of someone dying on screen.
As for the killer, I think it has to be either the Martell's, Tyrell's or Cersei.
My reasoning behind thinking its Cersei is because she doesn't want her family to lose control of Westeros. Joffrey was obviously a mad king who would easily be influenced by the sly Tyrell family. Also when Tyrion picks up the bottle of wine to pour for Joffrey you can see Cersei in the background which may be foreshadowing something. Also, didn't Cersei tell that guy (I forgot his name) to give all the leftovers to the dogs and said she was queen? At first I didn't think much of it just that she was a cunt, but now that Joffrey is dead she resumes as being Queen regent of Westeros I believe.
I don't know just my 2 cents. I'm probably so wrong.
But crazy episode.