I would say Monster Hunter, but now that it is sharing the franchise with the 3DS, I'm not sure it will have the same impact.
But that said, all it takes it one title to ignite the console, and it could come out of nowhere for all we know. I'm sure nobody expected Monster Hunter would single handedly revive the PSP in Japan, and nobody expected Brain Age and Nintendogs to ignite the DS to the heavens.