iPhone/Android. Nothing more needs to be said.
Actually, something needs to be said. All of you Clint Eastwood "get off my lawn!" types, who think that Smartphones aren't effecting the dedicated handhelds, or providing equally entertaining experiences (for less money too!) are seriously out of touch with reality.
Vita is gonna have the same problems the 3DS is having now. Don't be fooled when Monster Hunter sells a couple million either, because that isn't gonna happen anywhere outside of Japan.