My list would probably be more or less as follows (from weakest to strongest, JC skipped, beacuse I never met him):
Former Human (cannon fodder)
Imp (can be nasty at some occasions, but still easy to dispatch)
Former Captain (they CAN deal more damage than sergenats, but since they are not guarranteed to hit, they're less of a threat in the end)
Former Sergeant (autohit attack and tendency to shoot you just as you pass that one barell in the room means PAIN... actually I'd put them higher if it wouldn't be for their low HP... if you plan on diffrentiating generatin lists for diffrent challenges they should get even higher for AoB)
Demon (strong, and decently fast but once you get combat shotty or chaingun they are quite easy to get rid of)
Lost Soul (more dangerous than demons, thanks to higher speed and tendency to run in packs)
Pain Elemental (quite easy to kill, but if you don't have a shotgun handy they are likely to swarm you with spawned souls)
Cacodemon (almost as strong as HellKnight but ranked lower beacuese they can't use medkits and armor)
Hell Knight (see above... will probably increase in threat level, now that thye won't pickup weapons)
Former Commando (plasma is nasty, but with their limited ammo they are less of a threat than arachs)
Arachnotron (as I said plasma is deadly :P)
Baron of Hell (HellKnight++)
Mancubus (they aren't THAT much of a threat if you have space to fight them, but on enclosed spaces they are dreadfull... plus they seem to have a lot of hp)
Arch-Vile (auto-hit attack and ressurection - 'nuff said)