Which RPG provides the most useful resources?
I'm not sure there are any better general-case resources that are RPG-specific (as opposed to history books or the like) than GURPS books. GURPS has an entire line of books that are nothing but general-case resources for various backgrounds, settings, histories, and the like. There are some books that are specific to GURPS particularly, but even when ignoring those portions, I'm not sure there are any books that are better written or better sourced than the GURPS back catalog.
I mean seriously: look at this.