The year it was put out of its misery, so 2017.

2015 followed by the launch year. The rest of the console's history may as well have not existed as far as I'm concerned, Other than something I could play Wii and VC game on, I don't recall one compelling thing for Wii U. I suppose there was the Wii Sports game, and that drawing app, I think those came out at some point in the off years. There was also Earthbound. I got Mario Kart 8, but I have literally played it more than 200X longer on Switch - as I put in less than 3 hours on the Wii U version, so I consider Mario Kart 8 as a Switch game that just had an early version on Wii U that no one really wanted to play.

At the same time, should Xenoblade Chronicles X come to Switch, I imagine it would be a FAR more desirable experience, being it will be on hardware I actually like using.

2015 was okay too I guess; certainly far from great, but with Splatoon, Fatal Frame 5, Xenoblade Chronicles X, Yoshi's Woolly World, and FAST Racing Neo, its annual lineup was at least passable.

I consider that although 2014 had bigger games (Smash, MK), 2015 had better games, or at least, those games were something special, those were games that gave potential costumers a reason besides Mario to purchase the system. The wonderful E3 2014 presentation was mostly a 2015 lineup. Also, in a more general way, the best year in term of sales for the console was 2015.

Edit: plus I see a lot of people seem to dislike Smash Wii U.

The reveal of BOTW and Captain Toad were also key wins for E3 2014 for me, and those weren't 2015 games. For me, while I did really enjoy games like Splatoon and Fatal Frame, nothing in 2015 was quite up to the sky high bar of Bayonetta 2, Mario Kart 8, or Tropical Freeze.

2014. Not only were there great game releases, but they had some great new announcements for games coming in later to look forward to.

I can swear Captain Toad is a 2015 game, or at least it was released in 2015 outside the US.

Now that I check it, yeah, Captain Toad was 2014 in North America but 2015 in PAL.

2014 was the year when I got a Wii U and when I bought most of my games but my most played game was Splatoon, still 2014 takes the cake.

2014 was the high point, after that each year was just worse and worse.