Ouch this is hard. Had you asked me last year, I would have clearly said Mario Kart 8. Now I'm not entirely sure.
I just started Xenoblade Chronicles X. That could definitely take the title.
Haven't yet started Bayonetta 2. Actually still playing through Bayonetta 1.
Just got Splatoon. Certainly a contender.
Smash is of course a top pick. It would be an easy winner if these other games weren't so freaking good.
Loved Wind Waker HD. My first Zelda game, but I don't think it is the best Wii U game.
Have only played a little of Tropical Freeze. 2D platformers are not my thing, but I have really enjoyed what I've played of Tropical Freeze.
So I would then narrow it down to Mario Kart 8, Smash Wii U, Xenoblade Chronicles X, or Splatoon.
Also, are we only discussing Wii U exclusives? Because if not, games like Shovel Knight are up for discussion.