I know this isn't exactly an "undiscovered game," but about six months ago I got around to playing Captain Toad: Treasure Tracker and I'm not sure if I posted much about it. It's my favorite game I've ever played on the Wii U, and the level design is consistently incredible and clever. It's a cute, lovely, thrilling little game and if you give it a chance I'm sure you'll absolutely adore it. It can be tricky, but I honestly liked it even more than Super Mario 3D World.
Also for Wii U eShop, if you're looking for a really really cool experience that uses the Wii U's features in absolutely crazy ways, consider downloading Year Walk. It's a pretty short experience, only a few hours, which may seem like not much bang for your buck at like $7 or whatever it costs, but I'd say it's worth as much as $10 or $15. It's really great. I played that one when it came out back in October, so I've had some time to distance myself from that too, in case anyone wants to check bias haha. It's definitely a little on the creepy side, but it's not "jump out of your seat" scary--think maybe like a stop-motion Tim Burton film. Give it a try. Its use of the Gamepad is absolutely brilliant.