Always cracks me up when people put any of the Azerothian villains (i.e. Azhsara, LK, etc.) above KJ/Archimonde in power. I think people don't sometimes grasp how much stronger the latter two are than anything we've faced with the exception of the Old Gods. It literally took Nature itself to kill Archimonde, and Nature pretty much went Kamikaze to the max and detonated a nuclear explosion. Both Archi/KJ require so much power being channeled into a ritual to even be manifested on Azeroth that it generally requires wells of power akin to the Well of Eternity/Sunwell for such a feat to be attempted. Archimonde snapped Malorne's neck like he was a toy - MALORNE's neck, the most powerful of Azeroth's natural guardians - not to mention he destroyed Dalaran (the most powerful magic city in all of Azeroth) by building a sand castle and trampling on it, and his magic is considered to be second to KJ's. Illidan was so afraid of KJ that he literally fled to Outland and sealed all the portals, while he would try to make an army of fel orcs a la Bloodboil style for war in the inevitable reunion.
Don't get me wrong. All the big baddies we've fought so far have been powerful in their own right, and Azshara is probably the single most powerful of all Azerothian born baddies. But, none of them really compare to a fully manifested Archimonde or Kil'jaedan.
This post obviously does not take into account Old Gods or Titans. They are on a different tier, altogether.