Wii Fit / Health Pack, no question.
When this game arrives we might see a Wii craze that nobody could´ve ever expected. And together released with Wii Music and Wii Sports and Wii Play still selling 15-20k a week in Japan the Wii might even sell better than DS.
This is gonna outsell both Final Fantasy and SUper Mario Galaxy in Japan I guess... let´s wait and see but as someone mentioned: diet craze...