Probably Dark Souls. Even though it's cross platform and is released on PS3 + Xbox 360, the PC port is definitely just as challenging, if not more challenging with Keyboard + Mouse. The game is NOT forgiving at all, and you are pitted against "realistically" strong monsters. Makes sense to me though, as a lot of RPGs seemed to be too easy and too soft with death being nothing but a nuisance and monsters being easy to kill for grinding and leveling up. Dark Souls feels like death is around every corner and every death is a punishment to you.