From the Wiki:
http://www.ageod.net/agewiki/Replacements
"Priority of Units recieving Replacements
Replacements are not added deterministically, i.e from lower region number to higher, etc. There is a weighted randomization taking place. But there are factors influencing the chances to get replacements, that is why some units will often get them before others.
First there is posture, all passive units will be checked before defensive, before offensive.
Then for a type of posture, you'll get much more chances to get a repl if you are in a region with high supply stock...
And then, your chances will be increased if you are an 'alert unit'. Your chance is proportional to Detection Value x Hide value of the unit.
It may seem to be a weird criteria, but generally this is gets good mileage. Quality units, like rangers etc. will get repl before line units, and line units will get them before militia. "
So an "alert" unit in Passive Posture, in a region with a depot in an unbesieged city that also has lots of supplies gets replacements best. To fit the alert criteria, perhaps stack the sub unit (SU) that needs replacements with a small cavalry SU for high Detect Value x Hide value
I'm the 51st shade of gray. Eat, pray, Charge!