I only bought my Wii U right after Christmas, so I actually only own 10 games. That being said I'll give my top 5.
1) Super Smash Bros. - love this game with my wife and buddies
2) Mario Kart 8 - same thing here, good solo play as well
3) Hyrule Warriors - tons of content, fresh take on Zelda and Dynasty Warriors
4) The Wonderful 101 - goofy and heartfelt, overall fun game I can pick up at any time
5) Wind Waker HD - Honestly would be #1 but I feel its a bit unfair since its a remake
“What I say is, a town isn't a town without a bookstore. It may call itself a town, but unless it's got a bookstore it knows it's not fooling a soul.” - Neil Gaiman