This post is a part of the #RPGaDAY series for 2017 by David F. Chapman and RPGBrigade. For more information, see this post at AUTOCRATIK. I'm modifying per suggestions from S. John Ross as well as applying my own interpretations. Comment with your answers or links to your own posts!
Day 26 - Which RPG provides the most useful resources?
I'm interpreting this question such that "useful" comes out to "generally useful" as opposed to "useful for that particular game," mostly because that makes it easy for me to answer. I imagine this will have to be a very common answer among Gamers Of A Certain Age, but my pick for this would have to be GURPS. Say what you might about the system itself, the GURPS supplement game was always on point. GURPS sourcebooks are, by and large, excellent and comprehensive resources. Further, they generally get authored by people personally enthusiastic about the subject matter, which is what happens when you basically just leave an open call for pitches. The goal of each sourcebook was to present enough material to build entire campaigns around, and that alone meant that they could be mined for information and ideas to be ported to any game. But each being based on the GURPS engine at the core also meant that it was effortless to mix and match them into whatever setting mashup your heart desired. While it always remains my goal to assemble an extensive and varied RPG library, I tend to want to go for breadth of systems rather than depth. That said, though, if I went the other way and wanted to build a whole library around a single system that could inform the widest variety of campaign settings, if not outright support them, then I would do it with GURPS.