For me, it's a virtual three way tie between Pikmin 3, Super Mario 3D World, and Donkey Kong Country. They're all 10/10 must-haves in my opinion. All three exceeded my wildest expectations.
-Arkham City comes in at number 4. Fantastic use of the Gamepad combined with one of my absolute favorite 7th gen titles means it's a winner.
-I can't think of a 5th. I own a ton of Wii U software but none of them seem all that worthy just yet. Gotta spend more time with Splinter Cell and Monster Hunter, I guess.