What is your go-to army composition?

[Slaists]

depends on the faction I play. As Romans, for example, I tend to roleplay a bit: hastati, mixed with principes, triarii, just a couple weak cavalry units, few missiles. As a hellenic pike faction, I tend to go 4 to 8 pikes + infantry or cavalry for flanks (depending on the faction setup and tech development at time) + the best javelins I can get for firing from the rear ranks. Why so few pikes? Because even though they are powerful, they are very inflexible (as mentioned by hooahguy above) when playing against the AI. AI would try to avoid your pikes at all costs and go for your flanks en masse. So, stronger flanks is a good idea against the AI.

I tend to avoid ballista unless it is a walled city assault army (that one would be heavy infantry + ballista + javelins for helping to clear the breach). No ballista setup makes for more fun battles when attacking the AI. If the player has artillery, the defending AI tends to rush you unless they have their own arty.

Then there is the early game "clear barb towns setup" for hellenistic factions: hoplites + massed slingers for no-walls settlements; hoplites + javelins for walled cities ;)