At first I wasn't sure what "intellectual" was supposed to mean here. RPGs are intrinsically intellectual (as opposed to athletic). So I'm taking it to mean an RPG where the play is about thought over action. That makes the decision easy: Call of Cthulhu. I still have one of the earlier boxed sets from Chaosium which still plays fine for me. I like a good Call of Cthulhu game but horror games are hard to do well. And the Keeper really needs to approach it for what it is and not just run D&D with weirder monsters.