There are some ideas behind the design of angels and demons in Diablo series.
The traits of angles are union, law, equality, and good.
Conversely, demons are individual, chaotic, separated, and evil.
(There should’ve been more terms to describe their duality but I’m just too unlearned to know them :o)
In the game, except for the Council, every angel has the same appearance. All of them follow the laws of the Heaven; all of them move on for the sole purpose of “good”.
Demons act as their opposite. They possess various appearance, various abilities, and various groups.
We’ve often heard that some certain demons are minions of one of the Evils, but it’s almost impossible to find that each Archangel takes fully control and power of command of other angels.
High Heavens is a single city where all angels live together; each certain space that possesses the traits of the archangels such as Garden of Hope only functions to fulfill the aspect of those corresponding archangels; these places are open to all angels at any time.
Yet, the Burning Hells are divided into 7 realms, of which one of the Evils governs their own realm.
The angels treat each other as their brothers and sisters, but demons would gladly torture their own kinds for fun.
In conclusion, I believe the most essential idea of the angels must be equality, and so I will consider that the five archangels are mostly equal.
For the Evils, it’s really hard to say. As the ideas behind their design are chaos and distinction, each of them are exceptional in different powers.
If the criterion is strength in combat, then I would consider that Diablo shall be the winner.
Final List:
- Five Archangels (together)
- Diablo
- Five Archangels (individual), Baal, and Azmodan
- Andariel and Duriel
- Mephisto and Belial
Side Notes: Each archangel possesses different aspects, which might not be necessarily particular when ranking their combating strength.
Anyways, such list, no matter made by whom, is always loose since each of these beings has different powers and traits.
Edited: When it comes to plotting and cunning, however, Mephisto and Belial are the masters.
The two have different styles when doing such of things. Mephisto spreads hatred among everything, even his brethren. Though never described specifically, he may make other six Evils fight each other and sit aside to take all the benefits (no matter what may be).
On the other hand, Belial’s doing is quite… simple: Lying. In the novel The Order, the main antagonist became such because, through the doings of Belial, he was convinced that he’s the descendant of Tal’Rasha and that he was tricked by Jered Cain, who’s Deckard Cain’s ancestor. From then on, he turned from a Horadric seeker (It’s quite difficult to describe his position since he only contacted with the Horadrim due to his jobs as a no-name bookstore clerk) to a Horadric Nemesis, who, sadly, still believed that he’s doing good things to this world (and to his so-believed but wrong ancestor). After all things were settled, Belial turned his goal to Caldium.