Alright, I've given this a bit of thought, and have some ideas. Sorry, it's pretty long. Also, SPOILERS! Using characters from the DF universe, and excluding the Hero, here it goes:
*If Elemental Lords are canon to DF, then they are number 1*
Probably the most powerful foe we have ever faced, the SMUDD wielded the power of the World Saviour Dragon, the MS, the NBOD, the Ultimate Orb, and a host of Darkness Elementals. It managed to subdue the Sun, manabreak Warlic, AND fight the Titan World Destroyer Dragon at the same time. Only once we were able to acquire the Ultimate Bacon Orb (which is insanely powerful) did we have a chance at defeating it.
Destroyer of Somorah, the homicidal Infernal half of Warlic is most likely the second most powerful character we have met so far. With complete mastery of the Fire element, such that he could enslave an Avatar, I have theorized previously on the existence of connections between him and other Elemental Planes as well. He may share mana limitations with Warlic, but it is probably safe to say that his are a fair bit higher (after all, he DID pull an Alderaan impersonation). Not to mention his huge army of fire creatures. His sole weakness that we know of appears to be his fragile mental state, and vulnerability to psionic intrusion.
The Blue Mage. Generates his own mana, but if he exceeds his body's limit, he loses control. Capable of fighting the Avatars on equal terms, his mortal versatility combines with his Infernal strength to give him abnormal magical prowess. Potentially capable of resurrection (well, Nythera DID use his powers to do it, so...).
We don't know how powerful Kathool could be, simply because it has never woken up. All our interactions with it were merely periods where it roused slightly from it's slumber and became a fraction more lucid. In this state however, it TOYED with Caitiff, transformed the race of water elves into grotesque servitors, drove almost the whole town of the Locker insane, and nearly enslaved the Hero. Stated to be nearly undying, it's madness could potentially infect anything...
5. The Avatars
Guardians of Elemental Balance on Lore, these beings function to preserve harmony amongst the elements. Pure entities of their element, they are virtually unparalleled (except by those before them on them on the list). Their greatest weakness is to the opposite element that they represent, and they can also be weakened through instability in their plane. To most peoples of Lore, these beings are seen as almost semi-divine.
The Empress - Not much is known, but she could definitely be a force to contend with as an amalgamation of unaels.
The Chaosweaver God of Nightmares - Is it real? Is it powerful?
Ascended Vaaloirr - He certainly talked the talk. Shame about the soul-eating though.
Isathaara - When the TM saga is complete, we can address this being's power-level. Seems to be capable of enveloping Lore in ice though, so probably pretty strong.
Zeuster/Arester - Said to be literal gods, we haven't seen of lot of what they can do.
6. Sepulchure (before losing most powers)
He defeats the Hero fairly easy, repeatedly. Wielding both the NBoD and wearing the parasitic armour of a Doomknight, his strength is basically unrivalled through most of Book One. Even before being corrupted by Doom, I believe it has been said that he was King Alteon's greatest warrior, and after being impaled through the chest by the NBoD, he still had the strength to fight by the Hero's side to defeat the Darkness Dracolich. Other powerful characters fear him, like Drakonnan, Noxus, and Xan, and with good reason. Also, he commanded a gigantic army of the Shadowscythe. So, yeah.
The Doom-Weapon-possessed corpse of Serenity. Very powerful, it succeeded in corrupting Draco, shrouding Lore in darkness, defeating Baron Valtrith (with the Hero's help), and driving most of Falconreach almost insane. The sheer dark power it can utilize was capable of damaging Ash's sword, and was beyond the ability of Aegis to handle. Only when the World Destroyer use some of it's power against it was Caitiff overcome, despite having fought many of Lore's finest previously.
Wielding the powers of both Soulsmithing and Chaosweaving, the prodigy of Ravenloss created the Judgement Wheel, one of the most powerful artifacts in DF. Claiming to be god-like, he dispels Greed with ease after becoming annoyed that it used the Wheel. After losing some of his powers he still remains potent, annihilating foes in the Deep Void with contempt. Roirr is the only Fleshweaver that we know of, and his nightmarish powers are as effective as they are gruesome, letting him gain a semblance of immortality as he consumes souls and jumps bodies. When the two of them meet, Vaaloirr is made, possessing the Fleshweaving of Roirr in Vaal's body. The two later ANNIHILATE THE WHOLE MAGISTERIUM, in the heart of their power, in the center of their Empire. Weak to forbidden magics though, as Roiirr's soul is so corrupted.
Warlic says she is close to being his equal, and that is not easy. Apprenticed to the greatest Mage on Lore, she wields both traditional spells, dragon magic, and Void energies. She takes on and defeats the Decadere Eldest single-handedly, proves herself equal to the leader of the Creatioux, and strikes a massive blow against the Titan Doom Draco. She also fought and defeated Warlic (though he let her win).
10. Xan and Drakonnan
Our two favourite crazy pyromancers, both with some of the saddest backstories in DF. While Xan definitely appears more powerful later in the game, Drakonnan's possession of the Fire Orb granted him a period where he was certainly the top dog. Both have a powerful affinity with fire, and are masters of the element. Xan has defeated the Hero before, and it took the combined efforts of all of Dragonia's hero's to stop Drakonnan. Even as Alexander, Xan was still a powerful mage, becoming proficient at his opposite element to the point where his cryomancy could have defeated Warlic.
Elryn - No idea what his limits are, but he seems extremely powerful. (Might be a Shaking King)
True Mortal - See above. (Except the Shaking King thing.)
Nivalis - Seems powerful, but we haven't actually seen him in action.
Aquella - She says that if she gets her staff, she will destroy the Locker. Is she kidding, or does she have that firepower? Also happens to be the Aegis of the Water Orb.
Prime Elemental Dragons - Probably going to learn more about them at some point, but I refrain from judgement currently.
Dove - He blocked a direct blast from Caitiff, which is VERY hard to do. But what is he?
Note that this is not a comprehensive canonized list, and that a person's ranking is not the be-all-and-end-all. Many characters lack raw power, but use deceit or skill to prevail. (Ie. If Sek-Duat fought Nythera, it might be closer than you think. Even though he is not listed, his skill and manipulation make him a foe to be reckoned with.)
I actually could extend the list, but must take a break for now. Love to hear feedback, and any reasons you might have for disagreement.
**Looking at this after posting, I feel terrible about it's length. Sorry fellow forumites, my innate verbosity precipitates truly vexing quantities of loquaciousness.
***I do not take credit for the placement of Zeuster or Dove.
< Message edited by Steel_King -- 1/6/2018 10:16:07 >
The Harvest approaches
I am the Reaper
Sing a song of Crows...