The most intellectual RPG I own? Wow, that’s a weird request, but one I can definitely fill.
“Intellectual” to me means something that is self-aware about its intellect. Something that explores difficult questions without apology. An RPG like Tribe 8, that so clearly draws from a deep, deep understanding of myth and religion would be somewhat intellectual. An RPG that seeks a completely simulationist view of interstellar travel, while boring, would also scratch that itch.
With that, I give you Cavemaster RPG, an RPG about playing a prehistoric homo habilis. It uses a mechanic (Habilis) that does not require anything you wouldn’t have had on hand 2 million years ago– just rocks, sticks, hide, and maybe some language.
I think this is an intellectual game because, despite my not having played it (yet), it focuses on the actual historical “what ifs,” and (unless you get the Lost Valley supplement), does not make the mistake of putting dinosaurs into the human timeline. The setting presented is realistic, and the mysticism can remain just that– human mysticism, not wu-ju magic that actually works.
Cavemaster does take two main speculative liberties– there are four human “species” in the world, interacting together (as opposed to the two concurrent that we know of– Neanderthal and Cro-Magnon), and the game proposes to ask “what happens if the humans keep developing technologically during the Stone Age?” This lends itself to the Cavemaster invention of “stonepunk” culture, but there is absolutely no reason someone couldn’t stick to the four-species neolithic concept and tell caveman stories without the “punk.”
It’s also entirely possible that I like this system in part because I’m a huge fan of Land of Og, which is, quite possibly, the least intellectual RPG I own. That, and I have a very extensive collection of caveman and dinosaur miniatures.