(This is in response to the #rpgaday list of questions)
I’m not entirely sure what “most intellectual” RPG I own even means. Clearly *Aria: Canticle of the Monomyth* is the most pretentious RPG ever published in English, but do I own that or was it Rob Barrett’s copy?
I think, though, that I’m going to go with a different take on “intellectual”, and make an argument that good old D&D white box is the most intellectual of the RPGs I own. I mean, besides the fact that even understanding how to play requires a bunch of analysis of cryptic text. Unlike most of the games that came after it, OD&D engages the GM and the players on a purely intellectual level. Not that you have to be smart to play it, but that there’s nothing to do, no way to resolve situations or move ahead in the game except by thinking, trying to understand the fictional world, and making decisions based on that. Any puzzles that are encountered have to be solved by the player. Anything the players attempt to do outside of a sparse set of tasks has to be adjudicated by the GM based on what seems reasonable: there isn’t even a hint of a mechanism along the lines of pick a difficulty and roll a die, or rules to mold outcomes according to aesthetic considerations or play that emphasizes non-thinky things like performance, empathy (especially empathic understanding of your own character), and so on. You could use OD&D as the basis for such play, and it wouldn’t get in the way…but that’s because it said nothing about it. Everything that’s actually on the page is about play as an intellectual task. OD&D doesn’t even use the term “role-playing” anywhere in its text: the only uses of the word role are in reference to the character’s *function*, e.g. “roll three six-sided dice in order to rate each as to various abilities, and thus aid them in selecting a role.” (in context fighting man, magic user, or cleric).
Tales of the Rambling Bumblers 
A big part of OD&D’s engagement on an intellectual level has to do with how poorly organized and written it is. It’s an absolute mess as a system book, and the technical writer in me hates it, but I think that mess is also part of the appeal: it’s this hard to decipher mess of arcane information that, if properly interpreted, creates a wonderful gaming experience.
An astounding thing about Eric Holme’s Basic edition is that it is mechanically almost identical to OD&D, but it looks and feels so different because it is so much more decipherable than Gary’s. Having better layout editing didn’t hurt either.
I don’t think that’s really true. Not the bit about OD&D being disorganized and hard to parse, that’s certainly true, but I think that the intellectual nature of it extends to actual play even if you’ve had the rules distilled and interpreted for you into something fairly comprehensible (say by the DM). I don’t have Holmes handy to compare it to, but even by Moldvay Basic there’s GM advice about XP awards for good play (clever ideas and extra danger) and penalties for bad play (careful attention to alignment or shirking danger) that are completely absent in OD&D. I see it as part of the transition from Fantastic Medieval Wargames Campaigns to The Original Fantasy Role Playing Game.
The criteria you use, though, to define it as an intellectual game seem to be based on a detachment between imaginative problem-solving and game mechanics. Given that adherence to the mechanics of D&D is one of the first things that generally get thrown out the window anyway, a better case might therefore be made for an open-ended diceless system like Microscope.
A better case for ‘intellectual’ could be made on a literary basis for AD&D, taking into account Appendix N and the influence of the writings upon mechanics, implied setting, and implied gameplay as well as the mechanics, setting and gameplay’s influence on future writings.
I don’t own Microscope, so even if that’s so it doesn’t count for answering the question. I think there are games (that I own) that are way more literary than any version of D&D, but that’s not really what I’m choosing to examine here. Basically, though, I’m trying to make some marginally interesting hay out of a vague and (to me) not very satisfying question. There seems to me to be a real sense in which OD&D is proto-roleplaying and there’s something interesting about how successors ran with that and opened up the scope for other mental activities.
In that, I can definitely agree. Part of what fascinates me is that period in game evolution where the board-gaming elements shift and become vestigial.
OD&D is the time during which it is impossible to say what the game is. It is a board game. But it is not a board game. It is a mini’s game descended from Little Wars, but does not need minis.
Sometimes these elements creep back in in weird, and sometimes non-sensible ways, and sometimes they get actively expunged. Most all games since try to find a spot on the spectrum of mechanical/board game to pure theater of the mind while owing their roots a game that could encompass both but was truly neither.
Also, you should check out Microscope. While it’s a game unto itself, it’s great for world creation since, after 2 or 3 hours of play, you end up with a pretty fleshed out setting that can be ported any other game system.