This one is pretty easy to answer, though I have to give an honorable mention to Lords of Gossamer and Shadow. That game challenged me with its no-dice premise and came up smelling roses. But in short, it is another game that challenged my preconception of what an RPG is and had me asking for more. That game was FATE Core.
No statistics? Come on! A character made up of phrases and a pyramid of skills. What is this madness? I will not go into too much depth but this game was very confronting to me. I followed its design and also had in depth discussions over the system with Jon Henry, Cameron Corniuk and Robert Hanz who was the FATE guru of our group. Much of my discovery and ideas about the game can be found here and here.
I have tried to run FATE in person a few times locally but the system does not seem to resonate with the local players. This is a real shame as this is a really good, fast flowing game that instigates a collaborative story as opposed to the railroads of the past. Railroading in FATE is exceptionally hard to do and hence you need to spend time building a framework and allowing the player to tell the story right along with you. It is the epitome of learn as you play.
Get out there and grab a copy of the rules – the PDF is pay what you want – get it for free and then go back and give them some money if you like what you see. I hope you love it as much as I do even if it is a challenging system to learn. Keep rolling…