Which RPG provides the most useful resources?
Not sure that I like this question. It seems biased in favor of large RPGs that have lots of splat books, adventure books, etc. There are lots of little RPGs out there which come complete, or nearly complete unto themselves that don’t provide anything by way of resources that are completely left out in this particular question.
But OK. Not every question has to be applicable to every roleplaying game I guess.
Ars Magica has unbelievable resources. the writers are all very smart and very dedicated to the game. Many of their supplements read like Masters theses on a given topic edited into game form, complete with lists of references. The game has attracted and inspired a large number of Medieval historians and enthusiasts who have in turn showered the game with love which shines through in the number and quality of resource books produced over its five editions.
Appealing to a broader audience than Ars Magica, the Fate system has really blossomed in terms of resources since it’t most recent reboot as Fate Core (and Fate Accelerated). With numerous setting books now out, as well as toolkit books and compendiums of articles on Fate and how to tweak it, Fate gives roleplaying enthusiasts great options at every level of detail, from those who want to simulate a pre-published setting to those who want to create their own.
Ars and Fate are interesting to compare in terms of their resources (and design philosophy) because they are such opposites. Ars Magica has a specific setting and focuses everything on defining/broadening out that setting in very specific ways. Fate, on the other hand, is largely independent of setting and focuses on giving GMs and players the resources they need to apply the rules to a setting of their choice. Whichever you are looking for, however, there is no denying that these games have quality resources available for them.