Fengol: That makes some sense, but then why are Militia Legions in the middle, instead of cannon fodder at the front?
Ranking:
1 -
2 - Tracker Legion (Trackers), Med Tank (Wolfen)
3 - Tank Killer (Wolverine)
4 - Militia Legion (Militia), Infantry Legion (Infantry), Power Legion
5 - SP Artillery (Smyter)
6 - Officer, Clergy
7 -
8 - Artillery, Anti Tank Gun (ATG), Noble
9 - Relics
I don't really have a great answer, but my take in general on asking questions about "why is such and such like this", the thing is that this is still a game from 1995. So any answers you'll get are just speculation. I could think of rationales, but I don't it matters. Mods will change everything anyway, and that's where the fun is. Take a look at Emperor Wars 4.0! It looks great so far!
Also, another aspect of Rank that Luxor didn't mention, is that efs.ini specifies the rank_assassination, which means that assassin-type units will attack units of that rank first. By default, it's set to rank 6. So assassins' attacks will start with rank 6, then 7, then 8, then 9, while regular units' attacks start with rank 1, then 2, then 3, etc. The idea is that assassins can kill your Nobles and officers quite easily even if they are in a stack with heavy armor.