The reveal of the Persona 3 plot by Aigis to Yosuke... and the realization that he just badmouthed a perfectly decent and great guy all along. Yosuke's realization that he genuinely did make both Yukari and Junpei mad was pretty powerful that it turns his first confident stance into a crying wreck that not even Aigis and Labrys encouragement could calm him down. It's... pretty powerful.
Yukari's Midnight Channel dungeon goes both ways. For Yukari, it's when she's re-exposed in her worst time possible, which she's trying to forget, and played it up like an utterly obsessed bitch. Yosuke? Wow, Gut Punch just doesn't get any worse, to see that his crush has a flaw way too big than expected. They'll both need therapy after this trip. Seriously.