Only bad thing is that almost 2 years ago I had seen a spoiler of what happened in the episode. Tried in vain to forget it. Obviously didn't work so I was pretty much just waiting for it to happen. Good thing about GOT is that there's so many things going on that getting one part of it spoiled doesn't ruin your whole enjoyment of the show.
It was clearly Queen Margery, look how she stiffened when Joffrey took the piss out of Renly's death. The poison is in the pie, not the wine. Look how she force-fed it to him, she interrupted him abusing Tyrion because she knew it might distract him from eating it.
She loved Renley, this is her getting revenge Joffrey for destroying him. Now she perhaps has a claim to the throne? (Unlikely)
Tyrion isn't stupid enough, with Cerseis other children being too young, it would only boost support for Stannis. He also knows Cersei would use any excuse to execute him. Tywin controls Joffrey, thus he doesn't need to kill him. Sansa is possible, but I just don't see her having access to poison and bringing it with her in the vague chance that she touched his goblet. Besides, she wanted to leave remember.
Margaery never loved Renly she did it for the alliance, Loras was the one who loved Renly, the tyrells were pretty pissed off with the dwarf show thing because clearly the lannisters have no respect for their enemies. They would never have done something like this themselves and neither would Renly, Robb, Balon or Stannis.
After hearing the title of the episode I was worried it was gonna be Dany-centric, but I liked the storyline balance. It wasn't OTT but it helped advance the plot and set everything up for where it needs to go next.