Pikmin 3 is really good.
Monster Hunter 3 Ultimate is really good, but it takes some amount of commitment to get into.
NSMBU is pretty good, but you might be hesitant to play it if you played NSMB2 recently, despite NSMBU being vastly superior.
NSLU is definitely better than NSLU, but it is quite a bit harder.
Wind Waker HD is...well, Wind Waker in HD. It's definitely better paced due to the Quick Sail, and looks better than it does in screenshots/videos, but it is ultimately just a remake.
I've heard that AC4, Batman: AO, Splinter Cell, Deus Ex and Rayman Legends also run really well on Wii U, at least compared to their PS3/360 counterparts, having less glitches, particular game-breaking ones.