Title says it all. Mine is Zombi U, which i have completed twice, and racked up over 40 hours total. NSMBU is hot on its heals with around 30 hours of gameplay :)
Nintendopie Was obviously right and I was obviously wrong. I will forever be a lesser being than them. (6/16/13)
Can't remember the hours but it goes like this.
Bit Runner 2
Batman Arkham City: AE
I think my Netflix is greater than all combined!
3DS FC: 3969 4633 0700
Lego City (~50 hours so far) followed by BO2, NSMBU, Batman, Nintendo Land, and then Little Inferno (~5 hours).
Still have to finish Batman and get to MWU, AC3, and Darksiders 2 which are sitting on the shelf. All that's on hold until I get 100% on Lego City, 14.9% to go.