Sargeras is the top dude, has always been the top dude, will always be the top dude.
Arthas is a powerful necromatic death knight, powerful as hell from his merger with Ner'zul. I think I'd put him on par with the power of Deathwing + Demon Soul.
Deathwing is a powerful badass dragon who was driven insane (or was found insane and that played to their advantages) by the old gods. He is the reason the orcs were so powerful during the end of the warcraft 2. I'd put him just below Arthas.
Azshara, bitch queen of the Naga. Insanely powerful Naga Sorceress. She by herself is not as powerful as Deathwing but with her army she might be more powerful. We know she controls the Naga and controlled the Satyrs while they existed in Kalimdor back in the day. Her current powers are largely unknown as is the size of her forces... She warrents more research.
Kil'jaden is Sargeras's right hand man. He was said to be more powerful than Archimonde. Theres no real proof of this in lore however due to the fact that he doesn't fight. I'd put him at a questionable state of power though I'd assume Arthas could bitchslap him.
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"A Warlocks job is to bring pain and suffering to those who annoy him."
- Overlord of the Twisting Nethers, Azune.