The perfect formation is the one that works against the other army.

Spears in front of archers with flankers behind the archers and cavalry off to either side will get you a long ways, but not against someone who knows that formation like the back of their hands. When the enemy's mounted knights wrap around your back at the same time that the enemy's foot knights hit your spears, you'll know that was the wrong formation to use.

Playing STW taught me to respect Sun Tzu -- if you are an unknowable enemy then you will be invincible. Change things around and see what happens. I get crushed experimenting all the time but it always helps the next battle (where I get crushed again )

Tamur