Pikmin 3. I bought a Wii U because of that game. Pikmin is one of my favorite all time gaming franchises, and the 3rd installment was easily the best in the series.

The first game I bought (Not including Nintendo Land) was Ninja Gaiden 3. No game really sold me on the system, it was mainly because it was Nintendo moving into HD and I was intrigued with the gamepad.

This.

Really sold me on the concept and Nintendo's direction with the system. Now if they could just reveal their 2014 line up...

Right now, not much, my Wii still makes me happy.

When X releases I'll buy a WiiU, and pick up a few quality games (like Pikmin 3) at that point. Untill then, I'll work on my Wii collection.

Zelda on wii u that's not wind waker.

It's not so much a single game as it was an event that sold me on the Wii U.

I talk about it at length in this thread here: UNITY - Nintendo & Wii U Finish The REVOLUTION.

It was Microsoft & their presentation of the XBox One at E3 that got me over my anger at Wii's premature deemphasization & got me on board with Nintendo's plan for Wii U (a continuation of the plan they began with Wii).

Once that was sealed Pikmin 3 put me right over the top.
And I bought my system within days of that game's release along with other must haves like New Super Mario Bros. U & Game & Wario.
NintendoLand being packed in was a boon as well.

Soon New Super Luigi U & The Wonderful 101 underlined that I made the right purchase.
The Legend of Zelda: The Wind Waker HD was icing on this ever-sweetening cake.

But Pikmin 3 was the game that drove me the most once XBox One awakened me to Nintendo's plan.
John Lucas

"X" when that game drops I will buy a Wii U

They were practically giving them away at my local store, so it was an impulse buy back in March. Plus I just 'knew' that Metroid was being developed by Retro (lol). Man I was butthurt after E3. It still got a lot of use because it's a cool new toy and can do some great stuff. My purchase is only really justified now though with all the great games coming out.

I'll get it for Zelda, X, and Smash and hopefully a good Metroid game as well.

Project X