It's all very situational, depending on your environment, size and location.
For example, if I'm playing a large country such as Russia, I'll have "self-sufficient" armies in the east, consisting of
CW+4 / 4 / CW
These armies are designed to control areas, and fight on their own. Because most eastern enemies are weak, usually a single army is enough to fight a war and take care of rebels.
However in the west, I'll almost always fight strong and numerous enemies, and my armies will never really fight alone, always in combination, so there I use
CW+4 / 2 / CW/2
These armies are smaller, allowing me to field more of them and control more strategic favourable terrain spots. Whenever they are engaged another army isn't far, so I'll get the full artillery line anyway.
Now when playing the HRE, especially during later stages of the game, I often use armies like
CW*2 / 4 / CW
The reasoning here goes as this, supply limits are larger and high forts are common, meaning I can stack more units without attrition. Also, in the HRE movements are extremely fast, and the last thing I want is one of my armies engaged by a strong enemy general who'll be able to decimate my infantry in days and start pounding the artillery, possibly stackwiping me. So here goes a lot of extra infantry.
When playing the Americas I tend to use much smaller armies in pairs, due to the low supply limits. The AI is terribly at keeping unnecessarily large stacks, especially in USA and Mexico.
And early game warfare is an entirely different animal, generally I stick with 1 artillery and as much horse as the support will allow, calculating for a safety margin of a few infantry regiments.
