Finally! Awesome episode.
I think it was the Tyrells. It doesn't seem like Tyrion is responsible. We'll see.
Damn, a whole week to go...
The way the shot lingered on the Lenna Tyrell with the chalice right in front of her just before, I have almost no doubt it was them, or at least her acting alone. Though if I end up being wrong about that, I will go on record stating that the one person I'm sure it's not is Tyrion. The look on his face alone when Cersei accused him on the spot proved that to me. Beyond that, if it were his plan, I doubt he'd plant himself so obviously in front of everyone where he'd end up being the most immediate suspect afterwards.