I suspect that the first thing that comes to mind is the United States and its allies due to military superiority and sheer numbers, but I'm not so sure about that. With the U.S. and company taking as much damage as they would have, wouldn't it follow that they would collapse into anarchy, maybe even into a loose conglomeration of individual states, while countries not involved in the war would become new global players? I'm not sure which states would be able to take advantage of a nuclear war, though. With the U.S. and its allies and the USSR and its allies, which countries would be able to take advantage of the situation in the fastest, most efficient way, and become new, key players in the aftermath?
Also, would the first side to fire in a nuclear exchange attack an ally country or go straight to the head?