The Pillars of Pelagia by Chris Doyle is my daughter's favorite module so far. It's got a lot going for it. A cool setting at the rocky seaside cliff lair of a wizard, lots of thematic puzzles, an intricate, interesting dungeon map, and a high-stakes boss fight. I ran this for the Riverport Rebels in person, around a live table, and had made all the puzzles into actual hands-on escape-room type challenges. When I ran it later for the Class of '81, I had re-invented all the puzzles into formats that could be worked in real time on the Roll20 virtual tabletop.
I mean, he wasn't wrong.
Anyway, after much argument, the seal was broken and the letter was found to contain an embarrassingly bad bit of love poetry. And after much mocking and hooting, the poetry was discovered to be an acrostic telling the Countess that the Duke was in trouble and needed her help.
SPOILERS: The players must get past an angry harpy to enter the keep, where a convoluted, multi-level keep features all kinds of interesting puzzles and challenges that will keep a party of adventurers on their toes, exploring and thinking. A trail of clues left by the wizard's pet faery dragon, Myricia, leads them on while telling them an increasingly urgent story about what's going on in the keep.
When I ran it for the Rebels, Arcadianus was still alive, and hiding in the underwater trunk that is in the water below this chamber. When the Class ran, it was closer to how it was written and he was dead already. Also, since the Rebels consists of two kids who were pretty young at the time we ran it, no cute little faery dragons can be burned to death in front of them. This is D&D, not How-To-End-Up-Paying-For-Therapy-Later.
Anyway, this module contains a mechanical submersible! It's supposed to only hold one person, but when young kids want to excitedly pile in and drive it back to Riverport, you let that happen. They named it the "S.S. Bumblecrab" and drove it home.