No, I am not talking about the super module released in 1987. I am wondering which of the original B series of modules is the best for teaching a Dungeon Master how to run a game, and consequently, going hand in hand, which module helps the players understand how to play the game. The answer is difficult to prove scientifically, so my following analysis is pure speculation and personal opinion. Feel free to comment if your views disagree.
First, I would like to eliminate some of the series quickly to get to a small list of contenders. B9 Castle Caldwell is a collection of adventures that vary in quality and do not present information in a way to explain the game. I would put this in a category of basic adventures but for semi-experienced game masters and players. I think quite a few of the series fall in this category.
B7 Rahasia is one of Tracy Hickman's early works that was lauded for eschewing traditional smash and grab dungeon designs. I have fond memories of playing this adventure back in the day, but I would definitely say that it does not hold your hand on how to run the dungeon. In fact, some of the opponents and traps can be quite brutal on an ill equipped party.
B6 The Veiled Society is an adventure set within the confines of a city, instead of a traditional dungeon. Politics, intrigue, and double dealing rule the day within the capitol city of Karameikos. The concepts are a bit advanced for someone just starting out to play.
Now things gets a little more difficult in the elimination process. My next series of eliminations is going to be controversial to some. I believe that including elements of wilderness adventure goes against the grain of what the Basic Set is suppose to be about. Camping, terrain, weather, rates of speed for overland travel, the free form nature of being able to go in any direction are all things that can add a level of complexity to the adventure (albeit slight).
I would eliminate B8 Journey to the Rock, B5 Horror on the Hill, and B2 Keep on the Borderlands for the overland part of the maps. It could be argued that the wilderness maps for B8 and B5 are dungeons in a forest, due to the way there are clearly marked paths (aka corridors). But for categorization purposes, I think it would be more appropriate to label them B/X as transitions between Basic (dungeon adventures) and Expert (wilderness adventures).
So what does that leave?
B1, B3, B4
B3 is a special case. There are two versions of the module; the original by Jean Wells, and then the rewrite by Tom Moldvay. The original version did have an overland wilderness map, but that was excised when Moldvay rewrote the module for publication. It is also interesting to note that Moldvay also wrote B4 and put together one of the most popular versions of the Basic Set (released in 1981, which also included B2 in the box).
B4 is a great module. I think it works well for a dungeon master in showing them a set of progressions to more complex play. The upper levels of the pyramid are keyed. The lower levels of the pyramid require some work of the dungeon master to flesh out how it all works. And then, it can move into more Expert play as there is an entire hidden civilization under the pyramid.
Even though I think B4 does a good job of starting off with basic play, and easing the players and dungeon master into advanced concepts, it does not provide much in the way of "hand holding" on exactly how to play. In the end, there is only one module from the B series that speaks in the language of "training", and that module is Mike Carr's B1 In Search of the Unknown.
Reading the NOTES FOR THE DUNGEON MASTER immediately sets the tone;
In general, this dungeon is less deadly and more forgiving than one designed to test experienced players.
It then goes into explanations about monsters, treasure, how to stock the dungeon; all valuable skills on how to run an adventure. This is not to say that B1 is the perfect introductory module for dungeon masters and players. It possibly could be improved by including things that were in the Basic rules set; such as an example of play. And even more hand-holding would be beneficial for the first few encounters.
But B1 was an excellent tool for beginning dungeon masters of the 80s. It certainly seems more helpful than the other modules in the B1 to 9 series. I have to wonder, though, if an introductory module such as this is even needed in today's climate of MMO's and video games. It would be interesting to interview new dungeon masters from the past decade and ask what hurdles they crossed and what they found helpful on the journey to running a game.