Lancel would be my first guess. Poor kid.
Second guess, the Hound. That would be epic, but somehow, I don't think it would end well for the Hound.
I think that Ser Gregorstain will win, and Cercei will be free, and on power again. But that's not the main point of Cercei's trail.
The main point is - that's my guess - that Dornishmen will think that Gregorstain is Gregor Clegane, and that Cercei tricked them. That will be
the trigger of Tommen's death, by the hand of Dornishmen in KL - probably, lady Nym, or lady Tyene, or even Dornishmen man-at-arms -
I can't remember where, but I'm positive that Cercei wanted to have a Dornishmen as a men-at-arms at KL and instructor to Tommen.
As many other Cercei's ideas, that will go perfectly wrong.
I think that Margaery's trial will go all wrong as well.
First, because, Cercei will be on power again, and she is dumb enough to do whatever she can to see Marg's head on a spike.
Second, it would fit Varys's goal - to end a Lannister/Tyrell allegiance perfectly well. He will see that Marg hits the spike, but blame will be
on Cercei. It doesn't get far better than that.
I want to apologize for any textual or grammar mistakes, I'm not a native speaker.