Tekken Tag Tournament 2: Wii U Edition
It was legitimately tough to decide which of the Wii U's many ports of existing games to include on this list, but ultimately we went with the ones that we feel best took advantage of their place on Nintendo's new console.
Tekken Tag Tournament 2: Wii U Edition adds a crapload of new Nintendo-themed costumes (seriously, they're hilarious), plus the new Mushroom Battle, Tekken Ball, and Tekken Supporters modes. Mushroom Battle is the best, adding various types of Mario mushroom power-ups that serve to make things a bit more chaotic.
The GamePad can also be used for a variety of functions, including costume editing and quick combos during battle, and the core fighting game is just as great as it was on Xbox 360 and PS3.