Although there are many adventures I wish I had written, top on the list would probably be U1 The Sinister Secret of Saltmarsh. The design is simple, the premise intriguing and fun, and the execution very close to perfect.
My first encounter with this module was on the shelf of my local hobby shop, AB Charles & Son, in 1981. The cover art by Dave De Leuw struck me as evocative of the kinds of horror stories I loved as a then 14 year old. I bought it posthaste, and returned home to pour over the text and maps it contained.
What I found was simply amazing! Here was a starting adventure with such great atmosphere that I was sucked in and began imaging all the wonderful possibilities to which this could lead. Not long after thoroughly familiarizing myself with the module I had my players roll up new characters and we began adventuring in the town of Saltmarsh.
My one wish for this adventure is that the town itself was detailed in the way that Hommlet was in T1. Still the players kicked around a “Theater of the Mind” town (with no map) and then headed off to the haunted house. My recollection of how this played out is that it was a slow building tension. The players were constantly on guard for the first evidence of “haunting” and scared witless from a “meta” perspective, knowing that undead are extremely deadly for starting characters. By the time the characters found the way down to the caves the tension was incredibly thick. I recall the exhalations around the table as the players started coming to the realization that the place was not, in fact, haunted.
Then the tensions was back as they nearly got TPK’d by the illusionist Sanbalet, “smuggler 2” and a couple of gnolls. Following that battle I remember there being a trip back to town for healing and resupply. Following that, the PCs returned to finish off their exploration of the house, cellar and caverns, and were subsequently charged with boarding the Sea Ghost. I recall there being trouble with the Lizard Men aboard the ship, but ultimately the characters persevered.
We never did continue with the rest of the series. I think it was some time before the other adventures came out and we continued on with the characters created for this adventure (with the exception of the paladin who died fighting a giant weasel on the way back to town). I think it is all the fun we had with this adventure that really makes me wish I had written it.