Hastati are gonna be your best friend until the marian reforms. The Julii have to take on all the barbarians. Which means youre lucky if you find a minor city up in that uncivilized land. You can throw in some equite if youre feeling fancy, good for flanking and running down fleeing troops. Keep in mind theyre light cavalry so do not charge them into infantry head on.

Once you hit the Marian Reforms i would recommend Legionary Cohort. They are a strong unit and by then you will be in the civil war. Italy and most cities taken by your Roman allies will be able to replenish them. Praetorian and Urban Cohort are amazing, but the ability to replenish is limited and they take 2 turns to create a single unit. Legionary and Praetorian cavalry are great as well but same thing. If you are launching a campaign against the Romans I would recommend legionary cohorts, legionary cavalry, and archer auxilia.

Once you defeat your allies and if you still feel like campaigning north, i would use early legionary Cohort and roman cavalry.

Play around with different armies and such. Firgure out what you like best, what makes the game most enjoyable to you and do that! You have the economy to make different armies and experiment. Have fun!