To me it is the amount and quality of troops. War is a game of numbers he who knows how to utilize those numbers win. Based on Sun Tzu's passage it even mentions in when to attack an enemy army and how to react with the numbers of the opposition. The only advantage modern armies have over ancient and older ones is one, firepower, technology and next is the C3(Communications, command and control). Still like any model of armies, it needs numbers to succeed in its goal.
Let me explain further, numbers is really a key in winning wars. The Axis in WWII was defeated because of the superior numbers of the Soviet Union and the Allies both in materiel and manpower. You may kill a lot of enemies with your superior tactics, but if you do not have a plan and our resources are limited you are only limited to signing peace treaties with your enemy. This is what Japan intended to do but they failed and lost when the U.S. continued to fight on.
Economy is helpful but then again if your country is in the front line it is not going to help. If the enemy harass your country's economic activities without the defense of your army then it is futile. Your economy will turn into dust.
That's why in CoW games it is preferable to have at least a sustainable economy but focused on a leaner military. Controllable, maneuverable and lastly numerous enough to deal with my adversaries. Unless you go solo you don't really need a very large army. I cannot imagine going solo against two to three coalitions with 5 members each. You loose the numbers game that way.
That's why for me the quantity and quality of troops matters first to economy, reputation and others.