Balance Board has already a great headstart and is still selling (even supply constrained) so it will be hard for the others to compete (even combined).
However, Vitality Sensor has the most potential to win as it is likely to be cheap and thus more easily bundled with games needing it (as others have pointed) which might allow it to sell enough to put the trio VS+Move+Natal above the balance board.
This is sales expectations so it has no bearing on my perceived quality of each peripheral.
Personally I would like Nintendo to do a EEG Sensor and bundle it with a Wii Relax+ as I have little difficulties calming my mind but once the adrenaline kicks in it can take a minute or two to calm my body so I would probably kick a lot more ass in an EEG game.
"I do not suffer from insanity, I enjoy every minute of it"
