I can't choose one, sorry. It's a two-way tie between:
Mario Kart 8: the reason I initially bought a Wii U, and boy I'm I glad I did. Vibrant, colorful and animated worlds enhanced by an amazing art direction. Super tight, responsive controls, even better with the godlike Wii U Pro Controller. Quite possibly the best DLC of any game ever, which gives you a staggering 50% increase to the base game, all at a value that I'm sure cannot be beaten by anyone. My absolute favorite soundtrack of this generation, so much that I actually had to import a copy of the OST from Japan. (It's the complete soundtrack by the way, includes all DLC tracks!). This game is pure, unfiltered joy condensed into a disc (Or a digital download...)!!
Super Mario 3D World: A lot of what I said about MK8 above applies here, down to importing a soundtrack from Japan. Unlike the OP, I don't have a problem with "floating platforms," and seeing them has zero impact on my immersion, feel of challenge, or enjoyment. How dare they have platforms on a platformer! Also, whether it's via soundtrack, levels, or other mechanisms, I love all the throwbacks to other games in the Mario series! (e.g., when riding Plessie, it's just a very slightly reworked version of the "slide" theme from Super Mario 64...). Certainly my favorite ever 3D platformer, and I'm pretty sure that if Rayman Origins didn't exist, this game may be my favorite platformer of all time, period!