With MGS4 having just released, this decision is really hard, but I'd have to go with the Wii at the moment. Twilight Princess, Brawl, MKWii, and SMG make a killer combo.
I would say PC, but the only PC games I've been into lately are rather old (Age of Mythology, the Battle for Middle-Earth II), with Supreme Commander being the only semi-recent release.
Edit: Oh yeah, BFMEII and SupCom are no longer exclusive. If you count those butchered ports as making the game multiplat....