Know what? I think I'm going to go into a lot of detail on this one. Lets consider some of the main aspects of war and see which comes on top.
Major Strongholds: Kalimdor gets this round simply because their cities are easily defensible and have better placement. Consider the fact that they have both coasts covered with at least one major city, and have Thunder Bluff in the middle. Also, each of their cities could be fortified to be impenetrable (Especially Darnassus.) Northrend only has the middle of the continent, leaving it vulnerable to attacks from any direction, and the Eastern Kingdoms only has the west coast covered. Pandaria would likely fall in last place in this round, unless they were allowed to consider the Wandering Isle as theirs. I would then put them in 2nd place.
Forces: I give this one a draw between The Eastern Kingdoms and Kalimdor. Since the cataclysm, both the dwarf and night elf population have been drastically decreased. However, The EK's have the Forsaken (which now has the Val'kyr,) and Kalimdor has the Tauren (who's strength and wisdom isn't given enough credit in WoW.) Northrend is far too weak still from the war against the Lich King, and Pandaria just doesn't have the population to go against any other continents. Also, Kalimdor would have a lot of elementals on their side!
Resources: This one is for sure given to Kalimdor. With the massive forests, wood would never been an issue. The orcs and goblins could produce massive quantities of war machines that could not be matched by the other continents. Also, Kalimdor looks like it would have more farmland to produce food for their armies. The only advantage the other continents may have over Kalimdor in this category is it's access to fresh water, but hell, mages can make that shit anyways.
Leaders: Again, this one would for sure be given to Kalimdor. If the races there put their differences aside and worked together, they would be unstoppable. Malfurion and Tyrande have wisdom and knowledge from living so long, and Velen's unlimited knowledge of the light would work well in their advantage, especially against their undead opponents. The strength of the Orcs and Tauren along with that would be an deadly combination.
Magic: This may be a difficult category. All of the continents have different magics they use in war. Whether it's the arcane, or raising the dead, it is still one of the largest contributors. Personally, I would say that Pandaria wins this because of the Mogu. Their ability to bend flesh could serve them well to create an army. I would rank the Eastern Kingdoms first here, but to raise the dead, you have to kill it first.
Geography: I would give this one to Kalimdor too. Eastern kingdoms would be my obvious choice, simply because of it's large bay areas that could be expended into massive harbours, but the only issue would be having the resources to do it. Kalimdor still has good areas that could be expended on by the water, but also has more habitable mountain ranges (Khaz'Modan is too cold) which could be used to their advantage. Also, if they were pushed back off of the main continent, they could regroup in Darnassus or at the Exodar.
Overall: Kalimdor would be my first choice to win. It's combination of strong forces, superior leaders, and plentiful resources would make it a difficult army to defeat. Eastern Kingdoms comes in 2nd place, with its large number of forces, but badly placed cities and less resources. Northrend and Pandaria would tie for 3rd. They don't have as many strong holds, and are both weaker in forces now.
TLDR: Kalimdor > Eastern Kingdoms > Northrend = Pandaria