1. Monster Hunter 3 Ultimate with 20 hours
2. Nintendo Land with 13 hours
3. Fifa 13 Demo with 12 hours
4. NSMBU with 10 hours
5. Tekken Tag Tournament 2 with 8 hours
6. ZombieU with 4 hours
(all according to the playtime app in the WiiU menu)
EDIT: The internet browser sits comfortably at 200 hours :D