Sure, you can argue that the orcs are evil -- and they absolutely were, back in the day. But when you start looking at the justifications for the orcs' actions, that label of pure evil comes into question. As for the Alliance, you can argue that the human race is a bastion of goodness and light -- but then you look at things like the Scarlet Crusade, at Benedictus' betrayal, and you begin to wonder whether the human race is inherently good or just as scattered as the rest of the world.
... Unless, of course, you look at the one place where evil characters always hang out: instances.
Instead, undead have so far dominated the category. Now Stubborn hasn't finished the study just yet -- next is an examination of Burning Crusade dungeons, then I imagine Wrath and Cataclysm dungeons will follow. And there's a fine line between undead and Forsaken -- while a lot of these bosses are indisputably undead, they don't fall under the classification of Forsaken, with some exceptions of course for bosses like Lord Godfrey over in Shadowfang Keep, among others.
As far as raid bosses are concerned, trolls come in handily in second place. But with classic dungeons, trolls are shoved down to fourth place, and in second place behind undead are ... humans. The more I think about it, the more I remember fighting mostly human bosses in vanilla WoW, particularly when leveling in the vicinity of Stormwind. Over on the Horde side, your first dungeon features orc cultists, demons, elementals and troggs, so at least there are some orc enemies to be found. But as you leveled through Horde territory, your next stop was invariably Wailing Caverns, which featured night elves.
All wondering aside, I'm really interested in seeing where the rest of Stubborn's calculations go, particularly after Outland dungeons, as the draenei right now are pretty much destined for sainthood in the Warcraft universe, if the trend of the totals we've seen so far continues. It does seem, however, that we can safely agree on one thing. Being undead? It means you're pretty bad.