I’m not sure I own intellectual RPGs. I probably do. I probably own several. Let’s see what I can come up with:
Microscope – It’s about designing and building whole worlds. I would argue it is not a RPG in Gary’s view or my view. However, it is a great tool to set up a shared world setting. I put Microscope in the same pile of books as Aria. Great products, but not what I am looking for with this topic.
Primal Order – Is an older product from Wizards of the Coast. The idea was to supply ideas on how to include deities and religion in your game. They supplied rules for various games in publication and received several cease & desist notices and maybe even a lawsuit occurred. The books are okay, but again, I don’t think this hits the nail on the head.
Nobilis 2nd edition – This is a physically beautiful book. The rules are light. The players take on the roles of sovereign powers where they represent abstract concepts (death, time, cars, the color orange, etc.). This game is likely the most intellectual in my collection.
However, I am going with Mage as my most intellectual. Mage requires the group to work through what is reality, to the individual and the masses. It also requires the group to work through spell casting, what is rote and what is improvised? When does one become the other? This game requires the most “thinking” among my collection. It is also a game that many people I know just never understood. It requires the players to get inside their characters’ heads and understand their view on magic and reality.