RuneQuest, by Chaosium, is probably the single best RPG book I've ever read or used. It is not without flaws and ambiguities, but I still run RuneQuest campaigns, and have done so for many years. The "Basic Role Playing System" by Chaosium eliminated the Dungeons & Dragons character classes and alignment system which always felt like bizarre constrictions on what was ultimately meant to be a free-form fantasy style of play. RuneQuest characters were defined instead by their physical attributes (like Dungeons & Dragons) and the skills they chose to study and train (unlike Dungeons & Dragons). The result was that a player could decide to have an eclectic mix of skills, unconstrained by artificial notions of what "fighters" did as opposed to "wizards." The combat system bears special note for its realism, though it comes at the cost of speed of play. The authors note that the combat system is largely derived from early SCA combat, and it shows, in isolating hit locations on a target, as opposed to the Dungeons & Dragons method of attacking an undifferentiated wad of "hit points" (or health, for the uninitiated).
The supplements, such as Pavis, Big Rubble, Cults of Prax, Cults of Terror, TrollPak, Griffin Mountain, and Borderlands, are all classics, and most have recently been reprinted. The supplements have a mixture of narrative storyline with linked adventures and stand-alone encounters, a consistently high quality of writing and imagination shown in devising a cohesive fantasy world, and the (unique for its time) notion that individual gods would have politically divided cults and pantheons of associated divine forces.