Pikmin 3 is the best answer for me.
Normally I consider the traditional controllers the best option to play Pikmin, but they're slower compared to the Wiimotes. So here comes the solution: the touch screen. It's just incredible, it's fast and almost inmediate, not even mentioning that it's almost perfectly accurate. I love these controllers. Also, using the Gamepad is fundamental when playing Mission mode to distribute all the leaders, otherwise it would be very tedious to pause the game, move the right stick and select the area you want. The Gamepad simplifies a lot and makes the game quite more enjoyable.
I would say Pikmin 3 is the only game that would work perfectly only on Wii U.
Aside from that title, I like how Arkham City simplifies gameplay as well, much better than pressing Select on the 360 version.
My bet with The_Liquid_Laser: I think the Switch won't surpass the PS2 as the best selling system of all time. If it does, I'll play a game of a list that The_Liquid_Laser will provide, I will have to play it for 50 hours or complete it, whatever comes first.