Although not a exclusive for the wii u at all, AC4: Black Flag was a great game. Also playing some Need for Speed: Most Wanted U (having a pro controller is so nice)
Oh boy, this is really hard. Pretty much all exclusives could go in on the list. But the game that made me go "Wow, what I'm playing is so beautiful that I wouldn't mind dying right now" was Super Mario 3D World.
Music: Masterpiece. Gameplay: 100% fun. Story: Simple and yet interesting. Simplicity works. Level Design: Perfect. Just perfect. Graphics: So smooth.
Oh, and the game is only 1.7gb. So you don't have to wait 4 hours to download/play a 30+ gb game with updates and stuff. What the people on Nintendo do with their franchises is just unbelievable.