Traditionally the following would be appropriate but the standing armies that Birthright tends to use require greater structure than history might allow for.
High Constable - ultimately considered responsible for all castles and garrisons.
Marshals - logistics men for the deployment of troops, often functionally generals.
Field- Marshal - traditionally the Marshal in charge of the operatons in the field, on campaign.
Captain - in charge of a military company
General - the officer in overall control of a campaign/province. You end up with Captain Generals and Lt. Generals too, but that is all tricky.
Early modern military titles might apply in the field but Ensign feel too napoleonic for my tastes.
Captain, Lieutenant, Ensign/Cornet, Sergeant, Corporal.