The Wii Vitality Sensor will sell the most. People have gotten motion controlled gaming on Wii for 3 - 4 years now. What they haven't gotten is something designed to relax you, something dealing with a very common issue in the world: stress.
It's a pretty big opportunity if you think about how many people world wide are stressed. I can see it being as big as Wii Fit if the software and marketing are both solid. I can't say the same for Natal or Gem.