This is kind of an easy question, because I've only backed one RPG ever. Blade Raiders.
By Grant Gould, an artist for the Cartoon Network animated Star Wars series, created a fantasy RPG that is so totally divorced from the Tolkein conventions and Gygaxian mechanics of Dungeons & Dragons that it really can't be called a "fantasy heartbreaker." I've talked about the game elsewhere, and I would still like to try it out sometime. It would be an easy game to convert into playing a campaign in the world of the Nickelodeon series Avatar.