Who is the most INSANELY evil (not most powerful) Sith?

Vitiate. By far.

Originally posted by Stigma
Agreed. Nihilus was more like a "midless" evil-doer. Those who are cold and calculated take the cake, hence Sidious and Vitiate.

Exactly what I think.

Nihilus was a regular dude who turned into an abomination due to the actions of others. Vitiate and Sidious were born evil and they both lacked any human emotion other than greed and fear.

Originally posted by Sinious
Vitiate and Sidious were born evil and they both lacked any human emotion other than greed and fear.

Originally posted by Emperordmb
Anger...Hate...?

Meh... they more or less had control over those feelings and turned them into weapons fueling their powers. Neither Vitiate nor Sidious never allowed those feelings to shape their plans. What truly triggered their actions were fear and greed imo.