For me, it's Path to Nowhere's "Dreamy Bubbles." The event touched upon very mature themes like addiction, loneliness, burdens of the adult life etc...It was told very well with story climax that can bring a man to tears. Most of all, it didn't end with a happy ending. The ending of the event was bittersweet and hopeful as the characters accepted their reality and moved on. Man, I wish every gacha story is like this. So, how about you?