Mario Kart without question. Before MK8, I never even CONSIDERED looking at Mario Kart for visuals. Now it's arguably the best looking game on the Wii U that isn't Wind Waker HD.
As great as Zelda U looks, Skyward Sword looks good too, especially for the Eii. So as far as transitional leaps, even though Zelda U will look better than Mario Kart 8 (and not my a lot), the leap isn't nearly as wide. Mario Kart Wii was a pretty mediocre looking Wii game.