How many people are actually using the Single Best RPG book they can think of?
Great question!
(My answer: to the extent that I believe the "best RPG book" is the one I write for game I play, well ... ergo. )
Cheater!
Seriously, when this thread came up, I had to give it some serious thought. I normally have to turn my sentimentality gear on in my head, when I do, I can think of *decades* of love I gave to the 1e DMG, Palladium Fantasy 1e, Talislanta 2e, blah blah blah... but if they were the BEST... why am I not playing/using them currently?
And I don't mean literally, I mean in my current lineup of games that are my GO TO for whatever. That seem the logical answer to myself without sentiment. I have no interest in gaming for nostalgia - this is not to say that nostalgia is bad. But for me, I'm trying to maximize my chances at a team sport to bring my game to Superbowl Championship status, where that campaign is going to last years, and be talked about long after I'm dead.
Secondly, I'm not against using what is "old" - that's not a bias for me. I run MSH as much as anything else in my lineup of games.
That's why SWADE Core is probably my pound-for-pound response. I could do nearly anything with it on the fly, or run it sandbox across nearly any genre of game with minimal prep required, with fidelity (within certain parameters depending on the genre).