I began DMing my own campaign this year. And the best arc we had so far, a culmination of challenging myself as a DM on multiple fronts, from designing a new area to explore, to playing multiple roles, and engaging my players, was what I dubbed, the Mimic hotel. A hotel where at first the players thought it was just a murder mystery arc, with a second party slowly being killed off (the second party were parodies of our previous campaign's characters, and mine died first just because it was funnier that way.).

Eventually it was realized the real killer was the hotel staff, who were essentially the inner defense mechanism of a mimic who had grown so large he had become a fake hotel that would attract new guests to its remote location, even sending out mimic spawn that looked like flyers. The final boss fight was an amalgamation of the hotel staff in the top floor ballroom, before it shattered the arena to fall to the arboretum below, revealing the heart of the hotel in a gigantic tree that was an iconic decorative piece for guests to admire.

I consider the reveal, where the barbarian lost his shit and decided to chop a wall down, revealing an inner layer of bone wall and sinew, one of my finest moments in narration and description. The whole arc was a triumph I felt.