The DS.
Without the DS there would be no Wii.
Also, Nintendo's next handheld is pretty much a sure success.
With the next console it will all be up in the air again. Nintendo will have to make some more magic to have a repeat.
End of 2009 Predictions (Set, January 1st 2009)
Wii- 72 million 3rd Year Peak, better slate of releases
360- 37 million Should trend down slightly after 3rd year peak
PS3- 29 million Sales should pick up next year, 3rd year peak and price cut