Looking at reviews and so on....The answer will most likely end up as The Last Story or Monster Hunter. I just bought it and am about to play it, will repost when I can confirm that it truly is the best 3rd party game on the system.
Edit: I just noticed, why isn't there a poll option of The Last Story? And don't tell me it's not a third party game, Nintendo only published it, and even that it didn't publish everywhere. Please add. Thank you ^^.