That's right, kids, the 1st edition AD&D Dungeon Master's Guide. Let's go over what this book has to offer to support my rather obvious choice.
- Charts. And tables. Endless charts and tables. If you're ever at a loss during a game, the DMG has so many damn charts and tables covering a huge range of options that you can just start rolling and come up with something. NPC reactions, room descriptions, encumbrance, poison, traps, construction of castles, whatever. Anyone can make a book of charts and tables, but no roleplaying book has ever done it to the magnitude and competency of the DMG.
- Advice. No book has ever laid out quite so succinctly exactly WHAT a roleplaying game is and the role of the dungeon master within the context of that game. I think the concept of "game" is the most important part here; the DMG makes no haughty claims of performance art, nor tries to pass off D&D as some sort of exercise in the exploration of the human psyche. No, it talks about a game that people play for fun. Gygax takes his fun seriously, though, and I suspect most gamers do. He expects the DM to be a fair judge, but never to side with the players nor the environment. Situations are to be handled with consistency, but always at the promotion of whatever is most fun. It's a fine line to walk, ensuring players get hosed for being idiots yet keeping them interested in the game despite their stupidity. As Gygax states quite clearly, unless there are repercussions for failure, reward has no meaning. Excellent advice, for anyone running any roleplaying game.
- Information. I use this term loosely, but the DMG has so much fucking information it's ridiculous. Descriptions on how certain spells work, aerial combat, magic items, economics, demographics, ecology, how melee operates, effects of lycanthropy, an almost limitless amount of information. None of which is required to run the game, Gygax points out. The DMG is simply a guidebook on how things were done for Gygaxian D&D, a reference each individual DM can use as an example for the creation of his own game. There is so much information here, so rich in flavor and detail, you can't help but stop after reading each paragraph and wonder how it would fit into your game or what you'd do differently.
- That leads me to the last point: Inspiration. Even if the DMG is devoid of any meaningful use for your particular game, it offers inspiration, and that is more important than possibly anything else. I have never been able to open the DMG without feeling somewhat inspired about a certain aspect of a game I was running. Say whatever you want about "Gygaxian prose", but it certainly makes me want to play D&D after having read it. That, I think, is the true legacy of the DMG, why it is in fact the best roleplaying supplement ever: it makes you want to play.