BOTW is clearly the game that pushes the Wii U's limits the most. The scale is incredible, the foliage means it's pushing a ton of polygons, and it probably the most advanced physics system of any open world game. I'm amazed it runs at all on the Wii U hardware.
As for what's #2, that's hard. Games like Rayman Legends, Yoshi's Woolly World, Captain Toad, Kirby and the Rainbow Curse, and Child of Light are not technically demanding but have perfect art styles.
Mario Kart 8 is tops when it comes to attention to detail. Bayonetta 2 is the best at sheer spectacle. XCX may not be as advanced as Zelda, but it still looks mighty fine.