To determine the truly best, or, more accurately, WORST, villain, we must analyze the candidates and their qualifications:
Dark Willow was the way that she was because she was driven temporarily insane by the senseless death of the person who she loved above all others in the world. If she was demonstrated evil, it as an evil driven by the desire for revenge, which in no way makes her actions any less wrong, but they can at least be understood and forgiven.
Spike, A K A William the bloody was a killer, certainly, but it must be remembered that, technically, he was a predator who saw humans no differently than any hunter sees a game animal, even to the point of actively seeking out "trophy" kills (Slayers) in addition to his more normal prey. As such, while it makes him dangerous to humans, his activity is once more understandable. He kills to survive and to test himself against the most worthy of opponents.
As far as we know, everything that Warren did, he did out of sheer selfishness and disregard for the rights and welfare of others. He lied, cheated, stole, manipulated, and killed, for no reason other than that he wanted to, and he saw no reason why he shouldn't do whatever he wanted to, whenever he wanted to, and for no reason other than self gratification. And, it must be noted, he is the only one of the three considered here who was an actual, unaltered, "normal" human. That, above all else, makes him by far the most evil of the three. He didn't kill to survive, and he didn't kill out of a sense of dealing out justice for any known offense. He just did it because she wanted to, saw no reason why he shouldn't, and felt neither remorse nor regret for any of his actions. That, to me, is true evil, and he is the only one of the three to actually demonstrate it. So, my vote goes for Warren as the best, or worst, villain.
