Ok, this is going to sound utterly sad - but I'm away from my normal resources and don't have these memorized. Could someone provide a list (in order of weakest to strongest) of the baatazu sub species, along with descriptions of look and personality?
Please? Pretty please?
If you want to put it into article format for folks to be able to reference on the site as well so much the better - but I don't have an MM at hand to look these up in right now.
Thank y's kindly! That'll make it a *lot* easier to work on my nanowrimo from work. 
Actually, the abishai rank, from lowest to highest: white, black, green, blue, red (like the dragons), according to this page. I think my ranking was based on previous editions.
Again, thank you 
Paeliryons for some unknown reason bear an unncanny resemblance to what Mimi from the Drew Carey Show would look like if she decided to go Goth.
Thank God someone else said it before I did. 
Hahahahahahaha! That was a good one!
*snicker*
That's about as bad as the Gothyanki joke the other day.
Kind of like a cross between Mimi from the Drew Carey Show and Ursula the Sea Witch from The Little Mermaid.
That's why I made this page, although there are lots of unofficial baatezu there.
This list on the Wizards boards gives a different set of baatezu (I ignored the Legions of Hell ones there, except in the Ten Courts of Hell part).
Anyway, those lists allow me to answer questions like this without having to search my memory.
Anyway, the basic list is this:
LEAST BAATEZU
Lemure - melted petitioners
Nupperibo (slightly higher in station than lemures, though they're not actually baatezu) - mute, blind, deaf blobby things
Spinagons - little winged devils with spikes they can fire at people; like imps with thorns
LESSER BAATEZU
Abishai - kind of like imps, but big and multicolored. From least to greatest: black, blue, green, red, white.
Barbazu - Like abishai, but bearded and wingless. Stupid brutes.
Kocrachons - beetle-like torture devils. They're also in charge of promoting the lower castes with their profane surgeries. Most of them are eventually promoted into erinyes.
Erinyes - Beautiful angel-winged female devils. They use pleasure and agony to turn mortals toward oppressive and orderly evil.
Osyluths - skeletal devils. The police of Baator.
Hamatulas - wingless devils covered in sharp spines. Guardians of the first four layers (Gazra, the pit fiend consort of Fierana, is in charge of all of them). Faces of Evil makes it look like they outrank osyluths, erinyes, and even cornugons, but that's clearly bogus, as the other three ranks are all smarter and more wide-ranging. However, some hamatulas work as the personal guard of pit fiends, which is a prestigious gig that osyluths and erinyes can't match, and higher fiends sometimes use them as secret couriers.
GREATER BAATEZU
Cornugons - like huge versions of abishai, kind of. They're named for their horns, which are their most prominent feature. They get good gigs like leadership of entire armies or guardians of the Dark Eight.
Amnizu - short, pointy-headed, winged fiends. They rule forts where the River Styx touches Baator in order to guard it from invasion, commanding armies of lesser baatezu. They outrank cornugons, but they can't be promoted to higher ranks unless they're demoted to cornugon status first. The reason for this is that all amnizu are loyal to Levistus, and none of the other Nine trust Levistus as far as they can chuck a snowball at him. This, of course, only makes the amnizu more disloyal to everyone but Levistus.
Gelugons - insectlike bipeds with long tails, kind of like the icy bug equivalents of osyluths. They dominate Cania and are seldom seen anywhere else (but you can sometimes see them as elite troops on the Blood War front, or spying on Levistus in Stygia, or in Sigil on special missions). Cornugons who perform well are promoted into gelugons, and gelugons who perform well are, after 1001 years of flawless service, transformed into pit fiends.
Pit fiends - Like fiery, venomous cornugons who have grown into their horns. The leaders of the Blood War and of all non-noble baatezu.
OTHER BAATEZU
Okay, that's all the Planescape baatezu. There are a few more in 3e books:
Advespa from the MMII. They look like giant wasps with female human torsos, made of stone. They're probably roughly equal to abishai.
Narzugons from the Manual of the Planes. I like these guys - pale humanoids who ride nightmares and other fiendish mounts. The cavalry of Baator. Probably roughly equivalent to erinyes, I should think, taking the martial path instead of the path of the seducer.
Aratons from Sandstorm. They look kind of like hamatulas without all the sharp spines and with their skin flayed off from the sharp sand constantly blowing around them. They're lesser baatezu, but I'm not sure where they rank exactly.
Ghargantula from the Book of Vile Darkness. Kind of like what you'd get if you crossed a hamatula with a tyrannosaurus rex, only not nearly as pretty. I'm not sure where they should fit in exactly, either; I generally assume the higher the base intelligence score, the higher the rank, so they're not that high. I'm tempted to just say they're wandering Baatorian monsters and not true baatezu at all.
Malebranche from the MMII. They look like cornugons made of stone. They're real powerhouses, but I don't remember them being that bright. Amnizu ride them around like flying horses.
Xerfilstyx from the Fiend Folio. They look kind of like cornugon or pit fiend centaurs with the lower bodies of slugs. They live in the River Styx, which makes them batshit insane. I doubt they're part of the hierarchy - I decided they were the special servants of Geryon (as amnizu are the servants of Levistus), cursed and exiled with their master. They kind of look like him, only sluglike where he's snakelike.
Paeliryons from the Fiend Folio. They look like fat pit fiends in heavy drag, with lipstick and nail polish. They're secret puppetmaster types. Again, I decided they weren't part of the formal hierarchy - they're special creations of the Dark Eight. A pit fiend might be transformed into a paeliryon if it's exceptionally cunning and needs to have its identity erased for some reason. Then they serve the Ministry of Immortal Relations or the Ministry of Supply or whatever from within the shadow. I actually like them, though I imagine many people don't.
That's all the official baatezu I can think of. I'm not counting other devils who are definitely not part of the baatezu hierarchy, like kytons or bezekira.