According to Lester Brown, it's food:
Grain yields are beginning to hit a “glass ceiling” in many countries, Brown said, where farmers have already taken advantage of what science has to offer for improving yield. As more and more countries hit an upper limit on productivity, the world grain harvest will begin to plateau, even as demand for food continues to rise, causing a rise in prices. More worrisome, the global food market is vulnerable to external shocks such as prolonged drought. “We don’t have idle land, we’re flat out,” says Brown. “We don’t have [food] stocks. We’re living harvest to harvest. The question becomes, what if we have a major shortfall in the world?”
Brown noted that war isn't a "top five" global security concern, but that seems to overlook the fact that possible food shortages and food price spikes could very easily lead to war.