Number 1 is obviously Breath of the Wild, no arguing there - in the end it became cross-platform, but BotW was originally the Wii U exclusive Zelda, and for that I'm grateful. I'm recording my current playthrough of it on my Wii U, even, and I think it's a bit more stable framerate-wise than my Switch version!
Number 2, is Splatoon. The game has kind of devolved into a cesspool with time and the release of Spla2 on switch, but this was the first shooter I've played in a long time that I genuinely enjoyed. I love that you can be terrible at the PvP, and just ink the ground instead, and it's a totally valid way to play.
Mr. 3 is gonna be the surprisingly stalwart Super Mario 3D World. It was a great "classic" Mario game mixing elements of the 2D overworld games of old like SM3 and NSMB (on DS) with crisp 3D platforming. It's no 64 or Galaxy, but it's a damn fine game and I had a lot of fun with the co-op.