Nintendo Land - a lot of really fun mini games. Nothing will probably beat Wii Sports' awesome Tennis game, but there are still a bunch of great quality minigames here. There are certainly more quality games worth playing here than were available in either of the Wii Sports titles.
Zombi-U - oh my god, this game is amazing. Survival mode allows to be basically "one life" it & see how far you can get without dying. The other mode (I forget the name of) is the same game but allow you to respawn as a new survivor to pick up where you left off in the event that you die (a good way to complete the campaign story with much less frustration). Death comes swift and harsh in Zombi-U, but the game remains fun & it's cool delving deeper into the game to complete missions.
Mario-U - very fun classic Mario gameplay. I'm also a fan of the challenges that the game offers. If I had any complaint, it would be that I really wish I could view the records set by my friends in the challenges. Without their records, I'm simply trying to beat my own personal highscore or acquire the gold medal. Being able to play on both the television as well as the game pad can really free you up. Good stuff.
Sonic All Star Racers - Cool game with some awesome flashy tracks. Haven't played this online or with any sort of multiplayer yet, but I'm anxious to do so.
Wii U is easily my new favorite console, trumping the PS3.