The Holmes Basic Set I owned came with module B1 In Search of the Unknown by Mike Carr, better known as the creator of Fight in the Skies (aka Dawn Patrol). If I had to choose the one module that had the greatest effect on me as a referee, it's this module, hands down. The reason is quite simple: B1 was written specifically as an "instructional aid for beginning Dungeon Masters" and so it was. I learned a number of really important lessons from using this module -- and use it I did -- chief among them being this: rooms containing pools of unknown liquids are cool.
More seriously, B1 really was an excellent "instructional aid." What it gave you was a two-level dungeon already mapped out for you, along with descriptions of most of the rooms, such as the aforementioned "room of pools." The module also provides a thin backstory about a pair of possibly evil adventurers named Rogahn the Fearless and Zelligar the Unknown who used their orc slaves to construct a fortress they called Quasqueton (presumably because Mike Carr is a fan of Frank Lloyd Wright). They then disappeared while fighting barbarians in the frozen north, leaving behind their fortress and its dungeons for other presumably non-evil adventurers to plunder.
The real genius of the module, though, is that, although each room has a description, none of them contains any monsters or treasures. It's entirely up to the referee, using the D&D rules and assisted by some tables at the back of the module to place monsters and treasures throughout the place. This might seem like a small thing -- and it is -- but the salutary effect it had on me was remarkable. Mike Carr had done all the hard work by designing the maps and describing the rooms, but he left it to each referee to populate the dungeon as they saw fit. From the start, I felt like a "co-creator" of B1 and it filled me with a strange confidence that I might otherwise not have possessed. And of course it also made it possible for me to re-use the module, which I did many times. I played the heck out of it.
The dungeon itself is pretty straightforward, with a handful of memorable locations, such as the room of pools and a garden of giant fungi, that made strong impressions on me as a young man. Some of the descriptions are absolutely priceless, such as this from the chamber of Rogahn's girlfriend: "A small tapestry measuring 3' x 4' hangs on the east wall. It depicts a handsome and robust warrior carrying off a beautiful maiden in a rescue scene set in a burning village, with a horde of ominous-looking enemies viewing from afar. Embroidered in gold cloth at the top of the scene are the words, 'Melissa, the most dearly won and greatest of all my treasures.'" That Rogahn sure was smooth, wasn't he? The descriptions gave me good models for how to describe rooms of my own invention, as I eventually did when I had an additional level to Quasqueton later on. That's what B1 was all about: good modeling.
The module is also noteworthy for having an extensive list of pregenerated characters in the book, all of them with names. And what names! Tassit, Servant of St. Cuthbert. Kracky the Hooded One. Mohag the Wanderer. Sho-Rembo. Ralt Gaither. Glom the Mighty. Luven Lightfinger. Feggener the Quick. They were all terrific and many of them became PCs, if only briefly, in my earliest adventures. When a character died -- and die they did -- I could just ask a player to pick one of the remaining pregens and we were ready to go again in five or ten minutes, after they'd bought equipment. The back of the module also contained expanded rules for finding and employing hirelings, which could be used in conjunction with the pregenerated characters.
All things considered, In Search of the Unknown succeeded in its goals: it taught me how to make my own dungeons. Interestingly, the original version of module B3, Palace of the Silver Princess, followed the same model as B1, since it had lots rooms whose occupants and treasure were intended to be added by the individual referee. The published version of the module follows a more "conventional" format and, I think, suffers for it. I can't recall any other published modules that take the approach of B1 and I think that's a shame. While I'm sure some refinements could be made to its approach, the overall formula is a sound one and could go a long way toward teaching the fundamentals of dungeon design to a new generation of gamers.
Perhaps I have another project to add to my ever-growing list.