While stretching it out a lot, I wouldn't say it's the biggest twist. I mean, the moment he let himself get exiled, there had to be someone to either be exiled in his place or taking his role at school, otherwise everyone figures out Zero is Lelouch and shit happens (ie: Nunally gets it).
If you ask me, the biggest twist is when Kallen gets captured. At this point, I'm expecting something LoGH-like now, with someone playing Oberstein on Zero and telling him to not show favoritism. Lo and behold, Diethard does it.
Yet Zero's speech, and his action of turning around to fight for Kallen, that was so completely out of character I could hardly believe it. Hell, I wanted him to man up and save her, but to think he actually would...