At the game start a legion is...

asseni, leves, rorai, hastati, princeps, triarri.

So post marian I would have

1st cohort, Evocati, 2 Cohorts Refomata, 1 missile unit.
This with auxillia support, or with another legion is a good army, if it needs to be a great army, put three legions in and yo will win.

@ Bacchus They will be unable to feild these armies often, and they will only attack when you have many units.

If you like the battles, I strongly advice trying out SPQR, it is a very good mod.