"Why?" is the keyword. I want to learn about cool game systems and I want YOU to tell me why it's your favorite game.
My favorite game system would probably be Warhammer Fantasy Roleplay (2nd Edition). I love medieval fantasy, but there are some things about D&D/Pathfinder that I just hate (NPCs take hours to build and die after two rounds of combat, treasure / encounter scaling, constant game balance issues, etc.). Warhammer Fantasy Roleplay, on the other hand, is extremely simple to GM and doesn't require much calculations on the GM's part : if you want to make your players rich or poor, do it.
The best thing about the game, in my opinion, is how hard it is : most games end up focusing on survival. The combat system is lethal and unforgiving and there's always a little risk of losing your character to a very lucky goblin wielding a fork. It's one of the only games I've played in which your character can receive crippling wounds, lose an eye or a finger or become mad after receiving too many hits on the head. You don't want to run into fight in the Warhammer Fantasy universe, and that's what makes it great : players have to calculate the consequences of their actions, no matter their level. They can't "hide" behind their eight class levels like some people do in D&D. However, that doesn't mean you can't be heroic and brave : in fact, a character who bravely charges into the fray is even more impressive when everyone around the table knows this character's courage might cost him his life. In a dark, realistic world, any act of heroism shines even brighter.
And of course, there's the well-developed setting, the dark humor, the generally dysfunctional adventuring parties ("a rat catcher, a nobleman and a priest of Ulric enter a tavern...") and many other great things.
So, what's your game and why?