Best one-volume game, eh?
Difficult parameters, to be certain.
Take for example
Pendragon -- I love this game! The core book contains everything you need to run a campaign ... but it is not a campaign to many people's tastes and is very constrained in what it can do. Same is true for
Call of Cthulhu; unless people are really into the setting (and a very specific take on the setting), the game is of little use.
Paranoia XP is a little better in that regard, but still a single setting.
Oddly, one game with a single rulebook that seems to fall into that category but doesn't (at least for me) is
Over The Edge -- yes, the setting is Al Amarja, but the core rules are so simple, so transparent that you can adapt them to a host of settings. I have used OTE for the base setting, for superheroes, pulp adventures, Cthulhu-esque, fantasy, and about six-eight variants on the above. This probably earns my "one volume" spot. It is also the game that I have used to introduce more non-gamers to gaming than any other books, which strikes me as a selling point.
The old RuneQuest (2nd ed) was also pretty decent that way, though a bit oddly constrained. Still, I have a soft spot in my heart for it.
Ars Magica would probably also fall in this category, either 4th or 5th edition -- different takes on the same base theme. While all the supplements are certainly nice, the core rules are all that you need for a good game; I have also worked up variant games (not using Mythic Europe as a base) from this material to rather surprising ends.
World of Darkness (the current core-book version) has been mentioned and I think deserves some serious thought. The system itself is elegant and simple, easy to move to a variety of gaming situations without the need for angst or even vampires. That, to my mind, is the hallmark of an intersting one volume game.
So, a few disparate notions...
