China.
It may be a while before it overtakes rice (if it ever does), but the potato appears to be ever more popular as a crop, since it gives relatively high returns for relatively low input.
It also has some advantages: the current rocketing price for rice and wheat is thought to be more due to commodity speculation than to any underlying real shortage. But to be able to fill a warehouse with something that you want to hoard for a year or two, you need to know that whatever it is will keep. Rice keeps. Wheat keeps. Potatoes don't. So though potato surpluses can be traded, they can't be hoarded.
This can also be a disadvantage: the Biblical story of the seven fat years followed by the seven lean years couldn't have had the same result if the granaries had been filled with potatoes. So for safety, you would need a mixed agronomy: potatoes for the bulk, but some rice and/or wheat to store for the lean years. (And not to be grown as a cash crop that carries on being exported willy nilly: the island of Ireland is still rightly aggrieved about that policy.)
I've never yet encountered the potato in a Chinese dish (unlike Indian, where Sag Aloo and Vindaloo are strongly potato-based.), but I await recipes with interest.
