What units should go in an army?

[The diplomat]

I have built several armies over the many games that I played but my armies don't seem very strong. I expected that an army would be much better than the same units unstacked, but I don't see much difference. They either lose or still take lots of damage whereby I wonder if a single unit would have done much differently.

So what is the best combo for armies? Is it best to put all offensive units together, all defensive or a mix?

[Aeson]

The most important use for an army is to win a battle so that the Heroic Epic can be built. I like to put 1 Knight in my first Army if they are available, or 3 Horsemen if not. Units in Armies cannot be upgraded, and that is why I'd only put in as many units as you feel would safely win a fight. It is best not to mix unit types, as different movement rates will use the lowest rate for the movement of the Army.

[barefootbadass]

Well, after you have heroic epic and you have enough high production cities to no longer need your leaders for wonders, you should put infantry type units in it, 4 of them if you have the pentagon, all veteran or elite if you don't need leaders anymore. This will save you some headache moving large numbers of defensive units and they function almost exactly the same(actually the ai fears attacking them, so it may be better). Do not put ofensive units in the army, you are just reducing your number of attacks if you do that, and if they are mobile, you have low risk of losing them anyway(except vs. mech infantry).

Before this point in the game I would follow Aeson's advice.