Depends. In the early game I like to spam cheap infantry and crush what little opposition I have by sheer weight of numbers. 10,000 peasants and town militia can crush 1000 knights. The key is to surround them completely and charge like you're indestructible. Confidence when you are carrying a rusty pitchfork means having 9,999 other peasants frothing like mad dogs next to you as you bum rush 'well equipped' excuses for armies.

Make sure you have a decent general though, and for God's sake, dont get him near the actual fighting. That's peasant work. That armor is no good around a dead general.

In the middle game, I like to spam crusaders or jihadists. Nothing says "I love you" like a raging horde of bloodthirsty zealots. Make sure you adopt plenty of generals by rushing wandering rebel armies with town milita on fairly even odds and winning battles. Your family tree will start to look odd, but that's ok.

Show those peasants they have a reason to fight... they might become part of the Royal family for their trouble, or become a national hero!

In the end game, It doesn't matter what kind of troops you have. You have so many to choose from. And you are smart to keep 2 or 3 stacks within 1 square of each other... there's strength in numbers.

Armor and good weapons are for cowards.
Yes, it's cheap, but it works.

...OK fine... you don't want to fight like a horde? Very well, 'civilized' man, all you have to do is work on cavalry. Lots of heavy cavalry paired with lots of infantry spearpoints. It might be ancient Greek strategy, but it works.

Archers are a waste of time unless you're on a hill.