State your case. :)

Pikmin 3

I say pikmin 3 since the gamepad is so helpful in multi tasking that and the wii remote I get so many fruit first day in garden of hope :)

I guess Pikmen 3 but I never really use the gamepad's touchscreen for gameplay purposes. I mostly use it for off-tv play

Pikmin 3, Batman Arkham City, Game and Wario and Wii Party U. Not saying the last one is good, but it puts it to good use from time to time. The Gamepad is really underused, I actually kinda hate it right now, hate the battery life and the fact that you can't turn it off in most games, If I wanna play on the TV, why is it showing me the game on the pad! Let me turn it off! A-la-DKC Tropical Freeze!

Even after all this time, I still think the GamePad's usage in Nintendo Land is sublime.

Luigi's Ghost Mansion is brilliant. Even today the family will ask if we can play "that Ghost game".

Wii Party U in my opinion. I think the many different ways it's used for minigames, for determining your dice role, for the tabletop games, for the drawing game, etc., are really creative and surprisingly fun. Personally, I haven't found the gamepad to be that useful for "regular" games where it's often just used for the map or inventory. Unlike the DS/3DS, it's difficult to look at the gamepad while focusing on the TV as they aren't in the same field of view. I tend to play most games with my Wii U Pro Controller and leave the gamepad on the sidelines. But games like Wii Party U and Nintendo Land have used it well in my opinion. For me, it has been at its best when used for local multiplayer purposes. The 3 "tag" minigames in Nintendo Land (Animal Crossing: Sweet Day, Mario Chase, and Luigi's Ghost Mansion) are still the most fun I've had with the controller, 2 full years later. 

Well besides Nintendo Land I thought Wind Waker used it pretty well.

I am yet to play a game that shows me the gamepad was worth destroying the console for =(

Nintendo Land :-/ Lots of great ideas. Too bad few of those made it into full games.