Clearly (and I don't think anyone would strongly argue otherwise), though our in-game power increases with each tier of content,the "lore" strength of our enemies does not necessarily increase.
I only know a little bit about the lore, learned from old warcraft instruction manuals, a few glimpses at wowwiki, and some things I've seen in-game.
I modestly propose:
The strongest opponents we've seen in the world are Yogg Saron and C'thun. These are the Old Gods.
After them would likely be Kil'Jaeden and Archimonde, the two highest-ranking generals of the Burning Legion, ruling the countless demons of the nether in Sargeras' long absence.
After those, there is the Lich King.
Beyond him, what comes next? M'uru, the insane and corrupted Naaru? Malygos, the mad Aspect of Magic? Algalon, the entity of stars sent by the Titans to "re-originate" the entire planet? Perhaps Illidan? I didn't consider him that personally strong - he was more of a clever leader - his power came from his ability to amass powerful followers and find powerful artifacts.
Does anyone have interesting ideas on this?