2013

Lego City Undercover

Zelda Wind Waker HD

Pikmin 3

Super Mario 3D World

Game & Wario

Wii Party U

Wii Fit U

Wonderful 101

2014

Donkey Kong Tropical Freeze

Mario Kart 8

Hyrule Warriors

Bayonetta 2

Super Smash Bros

Captain Toad Treasure Tracker

2015

Kirby and the Rainbow Curse

Splatoon

Mario Party 10

Yoshi's Wolly World

Fatal Frame 5

Affordable Space Adventures

Runbow

XenoBlade Chronicles X

Mario Maker

2016

Zelda Twilight Princess HD

Pokken Tournament

Star Fox Zero

Paper Mario Color Splash 

Tokyo Mirage Sessions #FE

Mario and Luigi and the Rio 2016 games

2013 or 2014, it is tough, both were great years.

2014, its not even a contest.

2015, Splatoon and Xenoblade Chronicles X some of my top games of the gen not just on the Wii U :)

Mario Kart 8, Smash Bros. and Bayonetta make 2014 my only choice!
@OP you mispelled Game & Wario, just a heads up ;)

Also you need to add Tokyo Mirage Sessions #FE to 2016 :)

Oh man, this is hard. I think 2013, 2014, and 2015 are all eligible.

I need to think more on this.

Bayonetta 2....'nuff said.

I would say 2014. DKC:TF, Mario Kart 8, Hyrule Warriors, Bayonetta 2, Smash Wii U, Captain Toad.

Even though I think XCX is my favorite Wii U game, and I think Splatoon is amazing, that 2014 lineup is hard to compete with. In fact, I think that 2014 was the year Nintendo really came to realize that the Wii U would never turn around. All those games and only a 10% YoY increase on Holiday sales from 2013.

2013 has the best game with Super Mario 3D World, plus The Wind Waker HD as a bonus. Not much else though while 2014 is all-round much better. I'd say objectively speaking, for as much as that's possible, 2014 with Mario Kart, Smash, Donkey Kong and a Mario and a Zelda spin-off wins hands down.