I am a big fan of B1. You have a classic dungeon design perfect for the old style of mapping and exploring. The DM advice is some of the best in the game, superior even to the B2 advice, which cribbed heavily from it.
But what I really like about it is what everyone else dislikes about it. The map is empty and relies on the DM to fill it up, either from the list the pre-planned encounters included, or from the DM's own ideas. I believe that this is a better model for introducing a new DM to the game, because it gives them guided experience in adventure design, rather than doing everything for them.
Unfortunately, once Gygax saw how well the Basic set was selling, he quickly replaced B1 with B2, to get those sweet, sweet royalties. And B2 did everything for the DM except name the NPCs. And with the popularity of both Holmes and later Moldvay Basic Sets, that became the expected model for adventures.
But I sometimes wonder. What if he hadn't done that, and B1 became the model for adventure modules going forward? How might our whole concept of an adventure module have changed, from pre-planned content to something more free form and adjustable? Even if it was only applied to beginning modules for new DMs? Less plug-and-play, and more kits of play elements for each DM to quickly and easily make their own?