I have too many to name just one, so here's my top 3.
The IC death of my character's first friend in Karakura in late 2018. It was typical gang violence, he just wasn't as prepared as the other side who 3v1'd him with bats. The interactions afterwards between my character and his IC girlfriend's were pretty impactful, but at least I got a free minecraft gamer girlfriend out of it. (Nobody in our small group knew you couldn't be killed by bats until much later, but by that point it was too late to void. ƒ)
A rap battle between myself and another teacher in early 2019, (i forgot my character's name lol) and Haruta Vance. It took place after school at the 11/7, and we had a large crowd of students hyping us up the entire time. It started out as normal beefing, but then Vance confessed his love for my character, who then dropped absolute BARS about friendship and brotherhood as a means of rejecting him without embarrassing a friend. I was pretty bummed when he stopped teaching, he was the coolest.
A more recent one happened in early 2021, people might actually remember when Tomoko Miki tried starting a rebellion in the school after she was fired for the 2nd time. I ran in and out of school grounds, narrowly avoiding faculty members and police as I dipped into rooms to make occasional announcements telling people to rise up against the faculty. I wish I commit to it more instead of bailing out when I did, would have been fun to actually lead students to overthrow the school. I also would have liked college intercom, so I could have spread the message to college, too (who at the time would have been 100% down to start rioting in the halls.)
Honorable mentions go to a lot of my classes as Tsubaki Sakiyama, Tomoko Miki, and Asuka Deguchi where I'm definitely breaking at least a handful of crimes against humanity for the sake of comedy and entertainment. The dating simulator class where "we're all gonna get hot dates," the times I had a Thanos gauntlet and "snapped" people out of my room (via /class kick,) and storming the school halls with an army of kids pretending to be cats as they meow at people to get them to join the cult were all pretty fun.