Probably Mario Kart 8. I've spent more time on that game than any other on the Wii U by far. Love racing into the wee hours of the night, Roy as my noble avatar. Keep rocking those sunglasses, man.
I also feel like ZombiU doesn't get nearly the credit it deserves. I'd say it's one of my favourite horror-related video games, beating out pretty much all of the Resident Evil titles, and it makes really good use of the game pad.