Undertale, for sure... I liked it when the game was still relatively new, and I can easily recall asking everyone in my class and some kids in other classes "Hey, do you like Undertale?" to which they would respond "No, I haven't seen that show" and then I would proceed to dramatically explain how it's a game and not a show and then spoil the entire story for them in a very long and annoying narrative.
Once I drew a picture featuring a very well-known part of the game (I won't go into details, I think I spoiled the game for enough people when I was a dumb little kid .w.) and showed it to the kid next to me in class. That kid, being very well-known for snitching on his classmates for basically everything, got up to tell our teacher who, unfortunately, was regarded by both parents and other teachers as one of the worst teachers in that school building (she lost her job after I left her class and several people were celebrating). That night I came home to a month's grounding and got Undertale banned from the household over a picture that wasn't even that bad.
My old best friend from my good ol' days playing Animal Jam (now known as Animal Jam Classic, rip) was the one who introduced me to Undertale. She also introduced me to Five Nights at Freddy's, Tattletails, and some other fandoms as well, and she got me into roleplaying, digital art, and creating OCs. She taught me what a persona was and helped me learn to accept LGBTQIAP+ (for some reason I grew up in a school full of bigots T^T). She pretty much played one of the biggest roles possible in my life when I was younger and helped me to become who I am now, and when I was banned from even bringing up the name of an Undertale character, it was really upsetting to have lost one of the things I shared most with my old best friend.
Undertale was a very big part of my life when I was younger, and even though I deeply regret some things I did back then, it still means a lot to me, even if I find it hard to engage with the community these days.
This is sort of why I hope Just Shapes & Beats, Project Arrhythmia, and Purrfect Apawcalypse will never become popular. I like how the communities are tight-knit and welcoming, and it scares me to think about how there's a chance that someday those communities might fill up with little kids that make tons of bathroom jokes who are only there because it's popular and fangirls who do things that everyone regrets, including themselves eventually (I know this from experience, unfortunately... Eugh.)
Sorry for writing an essay on something I could've said in two or three sentences, I'm not the best at keeping things short. ^^;