I have a rather extensive RPG collection. And I've read most of them. But it is Burning Wheel (et al), Lamentations of the Flame Princess, and Dungeon World that I keep turning to.
My collection runs the gamut of genres and topics, but there is something about games in the fantasy genre that keep drawing me in.
Burning Wheel (et. al) is heavy on procedure and is so very nuanced. I feel as though the author(s) are doing their best to have a very detailed and exhaustive conversation with me. They manage to speak up to me, instead of down. And they are always challenging me as a GM, player, reader, etc.
Dungeon World simply asks you to ask questions…lots of them…targeted and biased, working to solicit narrative movement from your players.
Lamentations of the Flame Princess is both comfortable and so very different. As if there is an alternate dimension in which I have played the game from its beginning and it resonates across time and space.
There are certainly others outside the bounds of Fantasy, but I have found it is the “common vocabulary” of Fantasy (thank you Gary and Dave) that makes Fantasy the perfect genre to create these fantastic works that acknowledge their foundation but say “Hey, watch this!”