AnthonyCordova
By suffering, learning
Many of the major villains from both shows have their scary component, so I agree with many of the posts above. I also agree deeply with the others about Wolfram and Hart. They work from the shadows, and they feel like a many headed hydra whereupon cutting of one head only means another two take its place. You never feel like you're making any progress with them (in 5 seasons Angel arguably made no progress with them, arguably even after killing the black thorn circle); they are all-powerful enough that others fight their fights for them, sometimes frighteningly powerful people in their own right.