Since someone somewhere always seems to ask ...
here is a list.
NOTE: This list only includes Lore characters - and the most powerful at that... any others fall under these.
THE MOST POWERFUL (And probably evil) VILLIANS LEFT ALIVE IN WOW:
1. Sargeras
Why?: Former champion of the Titans, whose purpose was to seek out and destroy all evil in the world. But seeing so much evil he soon believe it could never be defeated. As doubt and despair overwhelmed Sargeras' senses, he lost all faith not only in his mission, but also in the Titans' vision of an ordered universe. Eventually he came to believe that the concept of order itself was folly, and that chaos and depravity were the only absolutes within the dark, lonely universe. As leader of the Burning Crusade he has devastated countless worlds and brought many species of demon and other under his command. While he has been banished from the physicial world temporarily, you can be sure one day we'll have to deal with this guy. And how does one kill a god?
2. Lich King Ner'zhul (now in Arthas's body)
Why?: I'm sure this will cause some controversy, but this is how I see it.
Though Ner'zhul was broken and his soul bound by Kil'jaeden, the Orc Shaman was given unlimited power by burning legion in hopes he would use the scourge as another wing of the burning crusades massive armies. However, Ner'zhul had his own plans. Over the course of Warcraft III, he managed to trick Kil'jaeden, corrupt most of Azeroth (the continent), and ward off all attacks by Illidan (who was working for Kil'jaeden) - finally concluding with the defeat of Illidan at the hands of Arthas, his campion and now "occupied body". I would assume the combining of these two has created a creature so powerful, that I would hestitate to put anyone but the Dark Titan himself before them. The point is even more valid in that Kil'jaeden realzing he could no longer control Ner'zhul had to get a new pawn in the form of Illidan.
3. Kil'jaeden the Deciever
Why?: One of the two Eredar generals of Sargeras's army, and his most loyal follower, Kil'jaeden is the de facto leader of the Burning Crusade. The tactical mind behind the demon army, he has led the corruption of countless races and people, including the Orcs and Illidan Stormrage.
4. The Old Gods
Why?: Somewhere beneath the surface of Azeroth the remaining old gods (2-3 left according to WoWwiki) wait. With the power to take on the dragonflights and even provide a challange for the Titans, the Old Gods have countless tricks and strategies to one day retake the world. According to the Shifting Sands questline, Malfurion hints that an Old God has taken residence in the Emerald Dream and that perhaps one is located in the Maelstrom. Who knows what forms these creatures will take.
5. Deathwing (formerly Neltharion)
Why?: Leader of the black dragon flight and father to countless lesser evils (see Nefarion, Onyxia, etc), he was driven insane by the Old Gods. In his time he has posed as a human many times, once to cause the downfall of the Alliance of Lordaeron. In fact he became so powerful that it took all the dragonflights to defeat him. Though his wareabouts are unknown, it has been hinted by the other Black dragons (especially by the one who flys in to the dragonmaw basecamp now and again) that he has returned, this time with a new plan to defeat the other flights and seek vengeance on the world.