i only own 8 games but out of that set i would say pikmin 3, dues ex and batman arkham city. it was also useful for sailing in ac 4

Pikmin 3. Aiming Pikmin and planning ahead with the map was very helpful.

ZombiU and Darksiders 2.

SURPRISE! It's an EA game : Need for Speed: Most Wanted U.

Solid Motion Controls with the Gamepad. Easy to navigate user interface. Actually useful 2nd screen functions.

Wind Waker HD was really great though too. The Wonderful 101 deserves a mention too, but it's UI is rather clunky.

Rayman Legends did it utterly brilliantly with the Murphy levels. Particularly the endless challenge missions with two people are fantastic. It's completely different from anything else I've experienced, too.

In a similar way, I also loved how it was used in New Super Mario Bros U and Super Luigi U. I have a friend who I play 2-D platformer games with, but after a while it becomes too difficult for her, and it removes the fun. With these games, she played as far as she could manage, and then swapped to the gamepad.

I also played it with 4 people who aren't very good at platformer games, while I aided them with the gamepad, that was a blast too.

 In general, I'm very happy with it when it's just used as a secondary screen, though. The information you get in Hyrule Warriors, for instance, is very pleasant, and avoids cluttering the main screen more than necessary.

I´d say ZombiU. That game integrated the Gamepad functions to the gameplay and the game atmosphere like no other.

Other games also made great use of it (in my opinion): Nintendoland, Rayman Legends, Pikmin (very, very useful for planning!), Lego City Undercover, Stealth 2, Deus Ex Human Revolution

Nintendoland - The Mario and Animal Crossing mini-games are among the most fun games I've ever played (and I started playing video games in 1973 with a Magnavox Odissey, http://en.wikipedia.org/wiki/Magnavox_Odyssey )

having read through all of the responses i have to say,.. this feels like a stealth troll thread.

The wonderful 101

Pikmin 3 is only game that I genuinely felt the gamepad was useful

ZombiU could have been awesome but they blew it. There is no point to searching in that game 99.99% of the time you get nothing.