What's a good combination for an army? Lots of infantry with cannons? Lots of dragoons and irregulars? What?
With HOD?
The consensus seems to be you want about equal parts of infantry and artillery, maybe a little more infantry than artillery. Artillery can fire from the second 'square', so having infantry in front of them is essential. The new 'recon' and 'siege' stats now make at least one Hussar (or cavalry) and Engineer essential. Guards can be added as 'shock' troops to give invading armies a little more punch, but shouldn't replace infantry. Irregulars can be used if you're playing a weaker power (like an unciv, or the Ottoman Empire) that only needs cheap troops to fight an unciv. If you reach the point in the game where you can build tanks and aeroplanes, aeroplanes can replace cavalry/hussar and tanks can replace the engineer. (Possibly, or add one to a group besides an engineer.) Never make the mistake of thinking having aeroplanes and tanks means you're playing Hearts of Iron all of a sudden and compose entire armies of tanks and/or aeroplanes, that's a recipe for disaster.
If you're playing AHD: Infantry and Artillery, and Guards should take the place of your Infantry eventually when you can afford it because Guards are just stronger Infantry. Maybe include two cavalry/dragoons/hussars because they can attack diagonally. Other than that, you should have many one-brigade infantry armies to occupy a lot of territory at once. (You can't get away with that in HOD.)
Hope more knowledgeable players can confirm/correct this.