How powerful is Kiljaedan ?

[Rathbourne]

Just as a point of note, Illidan, though he had a chance to beat KJ, he may have been delusional and he admitted it was a slim chance but he thought he could do it.

Azshara however, rivals the power of the KJ and Archimonde and may very well be able to defeat either of them in terms of power.

Cosmically speaking Lich King is powerful in the sense that the dead vastly outnumber of the living if the Scourge was ever just turned loose they would eventually run rampant over everything on Azeroth be morals or deamons. However, the Lich King as individual entity is not even in the team picture with KJ, Archimonde, or Azshara, or for that matter Illidan, at the time of the Burning Crusade.

Azshara does not rival KJ and Archi in terms of power.They were superior to her.

The Scourge could raise the whole of Azeroth as undead and they still wouldnt be able to take on the full might of the Burning Legion.

[Strategos]

Not sure if this was said Before, but it seems that Lich King is more powerful than Kil'Jaeden.

http://wowwiki.wikia.com/wiki/Illidan_Stormrage

Under Allegiances you can read this:
After the Legion's defeat, Illidan was visited by Kil'jaeden, who, while noting Illidan's track record with the Legion, offered him one final chance to serve them. He told Illidan to seek out the Frozen Throne and destroy it. Ner'zhul had grown far too powerful for Kil'jaeden to control and Illidan was to remove him from the equation in exchange for more power and magic than he could dream of.

Too powerful to control =/= More powerful than

Good try though, that's some cute narrative spinning you've got going on

Sargeras > Kil'Jaeden > Arthas > Illidan

That is the canon list. I would say Illidan started at the lowest point however and has progressed the most. With all the new lore foreshadowing it's very likely he will surpass Arthas at peak power soon. One day he might even rival Kil'Jaeden. I doubt he will ever tackle Sargeras's tier, but who knows.

[DemonHunter18]

Too powerful to control =/= More powerful than

Good try though, that's some cute narrative spinning you've got going on

Sargeras > Kil'Jaeden > Arthas > Illidan

That is the canon list. I would say Illidan started at the lowest point however and has progressed the most. With all the new lore foreshadowing it's very likely he will surpass Arthas at peak power soon. One day he might even rival Kil'Jaeden. I doubt he will ever tackle Sargeras's tier, but who knows.

I hope you mean LK Arthas > Illidan > DK Arthas

[Strategos]

I hope you mean LK Arthas > Illidan > DK Arthas

What's the point of bringing DK Arthas into this? That's just redundant.

Let's mention Illidan > Paladin Arthas too while we're at it.

[Wildmoon]

You simply don't bring someone like LK,Illidan,Gul'dan,etc into the discussion involving KJ or Archimonde. They are on different tiers of power.

[Timberwind]

Two simple words: Void Lords.
Literally the reason Sargeras is doing everything he is, he's scared shitless of them.

No, He's not afraid of them at all, he "fel" because he discovered a Titan egg (planet that was a Titan) and it was corrupted by the Void Lord Proxies (old gods) and in his fury, he destroyed the whole planet and was forced to kill the Titan sleeping there as well, because he'd rather wipe out all life than see more corrupted Titans. I don't think it was fear, i think its rage. He destroyed the rest of the Pantheon because he believes his Burning crusade wiping out all life is the only answer to the systemic corruption of other Titans. His Goal started as a noble one, in his eyes. He would rather wipe out the universe's life than to see more of his brethren corrupted, even if that meant wiping out the existing titans. Azeroth is a Titan on the verge of waking, and will be the most Powerful Titan yet. Which explains why 1 - Azshara was so powerful (the well of eternity is the blood of Azeroth) and why we as a tiny race, can even fight the Host of demons attacking us. We're living on the most powerful Titan in the Universe (as far as we know) her power, even minutely, is in every race that inhabits her.
Side note
Azeroth -> Sargeras -> Kil'Jeaden -> Archimonde -> Azshara -> Deathwing -> LK -> Illidan

Side note, every being of great power we've fought in the world we've had the help of Titan creations. (Aside from Illidan who is an FROM Azeroth, no i'm not counting outland bosses they aren't "major") IE, Aspects, Wardens of Ulduar, Wells of overwhelming power, Nature "gods" yada yada.

[Lionhearte0]

one its 20 times stronger... unless your mythic raids only have 19 members... also we will prob have a massive force to help us, like with archimonde we had khagar, gromash, and yrel to help us

No, 19 times as strong. That's why he loses to 20 mythic raiders.

[Zulkhan]

It's true. The created isn't necessarily less powerful than the creators. However, we also need to keep in mind that KJ / Archimonde can destroy planet easily (ref Velen short story - "Prophet's Lesson"), Arthas - on the other hand - hasn't been able to do anything close to it. Also in "Prophet Lesson", Velen saw a vision of a Lich King "even more terrible than Arthas & Ner'zhul" who succeeded in conquering Azeroth. Was the Legion, or KJ afraid of him? Nope. When the Legion came, they laughed and toyed with this dead army on Azeroth instead.

In summary, you are right that being created doesn't necessarily mean or imply Lich King would be weaker than KJ. However, other things - such as what I mentioned - did.

To be fair, is not really much clear if the bulk of the Scourge was even on Azeroth anymore. There's really no mention about the slightest strife between the Legion and the Scourge and the target of the Legion's mockery are undead Draenei specifically, clearly because of Kil'jaeden. That's odd enough. Those undead, along many others, could have simply been a leftover of the Scourge on Azeroth, turned into a dead planet where only undead beings exist. After all, with the whole of Azeroth conquered, the Lich King could have likely found a way to move on other worlds.

[Selastan]

Just below Medivh and just above Azshara.

[Braven]

Well my lore might be a bit rusty but Kil'Jaden has to be somewhat close to Sargerus. I'd put him at the strength of a demigod for sure, but that differs depending on which to talk about I suppose (Loa, August Celestial, Ancients, etc.).

To reference your question, I believe Azshara and Deathwing would also be considered demigod level, but I'd put Kil'jaden over both of them and not by a small margin.

TL;DR: Probably second only to Titans, Old Gods, Elune, and the like.