Unlike RuneQuest, I actually played a lot of Greg Stafford's other masterpiece, Pendragon. Its first edition, released as a boxed set, was published in 1985 and I fell in love with it from the start. I think it'd be fair to say that I am a huge devotee of the legends of King Arthur, but, until I got my hands on Pendragon, I never seriously considered running a roleplaying campaign set in Arthurian Britain. Partly this is because I'm a bit of snob -- shocking, I know! I have a very particular vision of the Arthurian legends and I'd not encountered a roleplaying treatment of those legends that came close to my own. It was also partly a certain amount of fear: I didn't think, despite my very strong views, that I could "do justice" to the legends of Arthur and his knights.
Reading -- and playing -- Pendragon changed all that, though. It is, to be frank, the most perfect out-of-the-box RPG I have ever played. In my many campaigns over the last 20+ years, I only ever recall house ruling one element of the game as presented (mass combat) and even then what I did was build on the original rather than completely replace it. As written, Pendragon walks an incredibly narrow tightrope, simultaneously presenting a very wide-open and flexible game and setting without flinching from its razor sharp focus on its source material.
I've run campaigns where the characters were stalwart companions of Arthur, venal, mercenary knights, and even opponents of the High King and each and every one of them felt like it could have come from the pages of Malory. The reason for that is Pendragon's excellent traits and passions system, which is a mechanical way to quantify character behavior. Traits are opposed sets of personality factors that define a character's feelings or tendencies; most have a moral dimension. Examples of traits are Chaste/Lustful, Energetic/Lazy, and Merciful/Cruel. Passions meanwhile represent strong emotions, such as religious devotion, love, hate, loyalty, and so on. They are the things a character feels most intensely and thus enable him to perform heroic (or villainous) deeds of great renown. Taken together, these systems made it easy -- and enjoyable -- to roleplay a character whose personality not only felt like one from Arthurian legend but also one quite different from one's own.
The other amazing aspect of Pendragon is the backdrop against which it is set: the timeline of Arthur's nearly 70-year reign. This backdrop provides structure and a vital sense of history to a campaign, ensuring that every character's actions are viewed in a context greater than himself. This makes it easy for player knights to do important, even legendary things, without having either to usurp the roles of famous knights like Lancelot or Gawain or to play second fiddle to them. Furthermore, this timeline is important because Pendragon is a generational RPG. That is, characters are assumed to marry, have children, and die, with their sons eventually taking over for them. Given the length of the timeline, it's quite possible that players might be portraying the grandsons or great-grandsons of their original PCs by the end of Arthur's reign. Until you've done such a thing, it's difficult to describe just how mythic it all feels -- exactly as I'd always wanted it to be.
There are innumerable other things that Pendragon just gets right, too many to list. It's a rare type of RPG that so perfectly emulates its source material -- and enables players to do so themselves -- without feeling constrained or artificial. In the last campaign I ran (over a decade ago, alas), I watched with amazement as several players portrayed knights whose personalities and actions were quite unlike their own and they did so without difficulty. That's all the more remarkable given the melodramatic nature of Arthurian legend, with its hotblooded feuds and declarations of undying love -- emotions that are difficult, at best, to portray at the game table without feeling like a bit of a goof (at last in my experience). That's never been a problem with Pendragon, as its rules are written in such a way as to "mechanize" this kind of play without reducing it to mere dice rolls. Again, it's difficult to describe this dynamic but I've seen it often enough that I can only say Greg Stafford really knew what he was doing when he made this game.
In short, Pendragon is a true classic, one of my favorite RPGs of all time. I look forward to the day when I can play it again.