I wanna know the general opinion of the community on which year was the best for Wii U in a general way. You can argue from any point of view: games, sales, 3rd party support, etc. Since Wii U was a controversial console and had a rocky lifespan, I find this topic to be quite intriguing. Also, which year do you consider to be the worst?

This is the general overview.

2012: Launch Year. Nintendo Land and NSMBU. Nice 3rd party support (CoD, AC3, Darksiders, FIFA) and some exclusives (which were meh). Decent sales.

2013: Low sales. Empty first semester. Good games (mostly in the second half) like Pikmin 3, SM3DW, TW101, Lego City, Wind Waker HD and Game & Wario. Still retained some 3rd party support (AC4, Ghosts, Need for Speed). The console got its first price drop.

2014: Meh sales. Excellent games like Smash Bros, Mario Kart 8, Donkey Kong and Bayonetta 2. 3rd party support disappeared. At this point the lineup as a whole was actually kinda strong.

2015: This year the console had the best sales. New great IPs like Splatoon, Mario Maker, heavy hitters like Xenoblade Chronicles X and games like Yoshi WW and Kirby. This year also had the worst Nintendo games ever in the eyes of many: Amiibo Festival, Ultra Smash, Mario Party 10 and Devil's Third. Nindies also exploded this year.

2016: Almost everyone agrees that 2016 was Wii U's worst year. It was a hard time to be a Nintendo fan back then. NX was around in the corner, Nintendo was focused on mini consoles and Pokemon Go. Terrible sales. Games that year were Star Fox Zero, Pokken, Twilight Princess, Tokyo Mirage Sessions and Paper Mario.

2017: Nintendo killed the console. It got the long awaited Breath of the Wild which technically is a Wii U game.

2014 easily.
Mario Kart 8, Tropical Freeze, Bayonetta 2, Captain Toad, Hyrule Warriors; best annual lineup the system ever got.

2014 by virtue of the fact Mario Kart 8, DKC Tropical Freeze and Bayonetta 2 are the best games it has other than BOTW. Also, Shovel Knight, Smash Bros and Biding of Isaac released that year. And theres Hyrule Warriors which I guess was okayish.
2015 was fine as well.

2014.

Fatal Frame Maiden of Black Water

Tengami

Child of Light

Shantae and the Pirates Curse

NES Remix 2

Donkey Kong Tropical Freeze

Mario Kart 8

Super Smash Bros

Hyrule Warriors

Bayonetta 2

Captain Toad

Shovel Knight

2017. Breath of the Wild, Mario Kart 8 Deluxe, and Pokken Tournament DX. Just what a lineup!

I was talking about good games.

The only years that can be considered good are 2014 and 2015. Every other year of its lifespan was terrible. 2014 was probably the high point. At the time people were saying that the Wii U's line-up was better than the PS4 and XB1's. That's the only time that would ever be said.

Hyrule Warriors is a solid 7/10. Not great but in the Wii U's slim pickings it was worth a buy.

On paper 2014 would have been the best year but, Mario Maker was my favorite Wii U game...so 2015.

Breath of the Wild was better than everything else combined but its unfair to pick 2017 because of that, so I’ll have to pick 2014. Even though Mario 3D World and Mario Maker were 2013 and 2015 respectively, 2014 wins on aggregate because of Mario Kart, Captain Toad and Hyrule Warriors. Didn’t play WiiU’s Smash, but that was also there. 2016 and 2017 beside BotW were complete non-years.