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?
Limbus Company: Canto 6: The Heartbreaking
The midpoint alone of this Chapter alone is so amazingly good. From the music (bless Studio EIM for their godly work), to the story (glory to PM!), everything was phenomenal. The bit after the final boss, where you're sent to the title screen and all that, was amazing. The story between Heathcliff and ••••••••• was Heartbreaking pun intended.
It's all just so damn good to experience, even more so when you recognize where it departs and connects to the inspiration material (the original Wuthering Heights book and story). It's a treat for the mind and soul. Poor Heath will achieve his wish one day.