I watched the wedding scene again, and I'm pretty sure that Margaery and Lady Olenna were in cahoots.
First, Tyrion has no motive. What does he gain if he poisons Joffrey? Sure he stops getting made fun of, but he doesn't really seem to mind all that much. Second, it doesn't look like he slips anything in--and he had several opportunities to do it before Joffrey dies.
Why I think the Tyrells smelt it/dealt it:
-Margaery gets super-excited about the pie. They set it up to seem like she was excited about the pie because it stopped the confrontation, but what if she was excited about the pie because she knew it was poisoned?
-If it is in fact that wine that is poisoned, it looks like she slips something into the wine glass when Joffrey hands it to her. (It's not clear enough to overturn the call on the field, but it sure looks like it to me.)
-Most important--Lady Olenna and Margaery both effectively use the "She Who Smelt It Dealt It" defense. Margaery is the one to point out that Joffrey is "choking." Lady Olenna immediately says, "Help him!" And then what do they both do? Turn their backs while Cersei and Jaime run to try and save the king. They smelt it because they dealt it.
Now, what's their motive? It's unclear--either Margaery is going to make an appeal to the people (who love her) that she is the real queen, or there is some financial motive (Iron Bank?) that I don't know about because there's no way I'm going to read 600,000 pages.
WonderMeal's Probably 100% Correct Power Rankings of Who Did It:
1) Lady Olenna and Margaery
2) The Dornish dude (too obv)
3) The drunk fool (not capable of pulling it off)
100) Tyrion
Last place) Sansa. (too incredibly boring)
