Each nation has something going for it, it all depends in my opinion on the defensive properties of each nation.
Limsa Lominsa probably has the best chance overall for several reasons. Obviously they have the greatest navy out of the three, and would easily blockade and eventually starve out their adversaries. In addition, they seem to embrace gunpowder more which would make a huge difference in a siege.
Gridania can win a defensive guerrilla war by utilizing the Shroud to maximum effect, however on the offensive I believe they are the weakest, with their population being on the lower end and the tools available to them.
Ul'dah is a wild card, they have the strength and numbers, however their own city is woefully unprepared to deal with an extended siege, especially when it comes to water. I assume Ul'dah would have to strike first considering their defenses would cruel quickly from within.