Really, I was thinking about how similar they are.
WARNING, WALL OF TEXT! IF YOU'VE READ D. GRAY-MAN THEN THERE IS NOT MUCH OF A POINT IN READING THIS. Thankyou.
Now lets compare Hollow and Akuma.
How they are made:
Akuma are made when a human dies and another human (usually a friend or relative) mourns for their soul. Then the Milleniuma Earl, appears and ask if he wants to bring them back, and so he does. Only have their soul bound to the Millenium Earl and for them to kill the mourner and wear their skin. (So they can stealthly hide in public and work sneak attacks on people of course)
Hollow are made when a human soul does not pass on and is usually still their for some sort of reason. After a few months, hate, rage, and all that bad stuff builds up and their soul chain breaks, becomes a hole, and they become a beast known as a Hollow.
Level 0 Akuma and normal Hollow: Level 0 Akuma are skeleton like creatures that will kill its maker and wear their skin. All they do is slice though really. Normal Hollow are weak and mostly do melee attacks, their probably stronger than 0 Akuma.
Level 1 Akuma and Gillian: Equal in power and intelligence probably but Level 1 Akuma are easier to destroy. Level 1 Akuma can fire purple bullets from their circular like bodies. These bullets contain a poison that will automatically corrupt the human body and turn it into dust. For exorcist (the people that kill them) the bullets seem easy to dodge. Normal Humans are easily killed. They seem very stupid like Gillian.
Gillian seem weak and stupid as well, its only bad when they come in swarms. They also have only one attack, cero. They aren't automatic kills but meh they're still pretty powerful.
Level 2 Akuma and Adjuchas: After a few kills an Akuma can go into level 2. Level 2 Akuma take many forms such as a demonic jester, a frog, a dragon, even something ridiculous as a playing card that can't do anything XP. They are also very intelligent, it depends on the human host. Unlike level ones they have personalities, some silly, some annoying, some insane in the mainframe, but most are just really cocky. They all have their own ability, such as shapeshifting, wind storms, sound, bubbles that seem to shribble up bones and make your arms and legs like string, whatever.
Adjuchas like Level 2 Akuma seem very intelligent and they also gain abilities, well not really but their shape and battle style seems to change. Not much to explain really, but I'd say their equal in power.
Level 3 Akuma and Vaste Lorde: Level Three, the most devastating of Akuma seem to take a dark knight like appearance. They still retain their abilities I think, but to a higher degree. Only two have really been seen in the series though, one used chains to have control over gravity with high almost DBZ level speed. The other had a good amount of strength and used threads to destroy the atomic structure of matter. These enhanced abilities are powerful on an astronomic level as you can see. I can't really compare them to Vaste Lorde though since we haven't seen much of them.
Now for the icing on the cake!
Noah and Espada: Noah, direct descendants of Noah. (yes the one in the bible, plus they travel in an ark ) They are a family of superhumans who work with the Millenium Earl of their own will. They have grey skin and a cross across their foreheard. They're are 13 of them, only 5 have been shown so far. (there is a 14th, but he's different) They each have their own abilities according to what they represent. (Noah of Pleasure, of Dreams, of Anger) Like the Pleasure to bend the space. They also have an "Awakened" form, like an Arrancars released form but its kind of a mystery. Unlike Arrancar they have no rank.
Espada, maybe not as powerful as some Noah since a Noah's power is just about limitless while an Espada's isn't. Espada and Noah both have special abilities and personality. They both seem to command hordes of Akuma/Hollow/Arrancar.
I bet most of you didn't read that, oh well. Here are pictures to help here.
The Millenium Earl
Level 1 Akuma
Level 2 Akuma
Level 3 Akuma
The Noah Road Kamelot XP