To no one's surprise, Pokemon
Sure, the DS install base in Japan is massive, and the 3DS community is growing at a rapid rate, but just imagine how huge the spike in sales for Nintendo's latest handheld would be if the latest two Pokemon titles were developed for the 3DS.
Never underestimate the selling power of those cute little monsters.
This officially makes Black & White 2 the best selling game of 2012 thus far, serving as the only title to surpass one million sales in Japan this year. Though, I feel like lumping both games into a single sum is sort of cheating.
This unstoppable games duo is set to launch later this year in the west.
[Source]