1.) Episode 4, 12:56 in; the scene with Misaki Tachibana bashing Yousuke Tateishi's face into the arcade machine. I was in awe of the scene (and still am when I rewatch this episode) because it comes out of nowhere; the conversation between Decim and Chiyuki is meaningful and the thud comes during their speaking - there's no break. I was so focused on their conversation that I didn't expect anything in the background to happen, much less that. And, you know, not that I'm condoning that, but I can totally see someone just losing it - their life is on the line, after all (rather, that's what they're lead to believe). It's a shame that the resolution was rather weak...
2.) Obviously, the skating scene, not much to be said here. A fantastic scene, with little spoken dialogue. I much prefer this type of storytelling, as opposed to random spoken exposition dump #427...
3.) Sometime during episode 8 or 9, with Shimada's sister being assaulted. The look on her face, being bruised... oh boy.