For me the three games that makes a good use of the gamepad are ZombiU, Game & Wario, and Wonderful 101. I still enjoy other games like Mario 3D World with the blowing thing, Nintendo Land, and some others.
I really like the gamepad.

The game didn't get very good reviews, but I have to say Game & Wario. There's some fun single-player games that that make good use of multiple screens and drawing with the stylus, but I think the Art game in Multiplayer is my favorite one. It's basically a Pictionary type game; the Gamepad has a word to draw, which appears on the TV as you're drawing it, and everyone has to guess. Whenever I bring my Wii U home for holidays, my family can't get enough of it. We always make fun of our hastily made drawings, it's always a riot. XD

They always call it Pictionary though...which makes me wonder why we don't have more board games turned video games on the system. Drawing games like this work pretty well IMO.

Close second goes to either Nintendoland or Pikmin 3, but I'm guessing those will be popular answers.

I just wish Nintendo Land at least has online so we can enjoy those good games with other people online.

WWHD. It's what I wanted from the gamepad. Make the inventory and clunkiness easy to use. Now they just need to not have it be drag and drop but rather just click the item you want in the slot. That would be much more intuitive with the resistive touch screen. Zelda U will most likely do this.

Hmmm, Pikmin 3 is a really good answer. Although I don't think the way I used the GamePad most of the time necessitated its existance -- when using it as a map, tapping the touchscreen paused the game, so it's hardly different from having a 'map' button on the controller.

So I'm going to go with ZombiU. Still.

Wonderful 101 and Rayman Legends

Fatal Frame V.

I tried it with the touch controls, it's alright but I prefer Remote + Nunchuk.

Having three completely different control options is fantastic in general though.

Black ops 2 : dual screen local multiplayer 

luigi mansion mini game in Nintendoland : awesomeness

Almost all games : off screen play

rayman legends: Murphy is great

Just off the top of my head.

oh, also wonderful 101.

I'd say Pikmin 3 and some minigames in Nintendoland.