Buu; Only Super Buu with a somewhat advanced intellect can even be thought for this. But, all the same it's a being made for evil. I don't consider something doing what it's created to do as evil as it has no choice but to do so.
Cell is essentially the same, with the exceptions of having an air of cockiness and maturity about him. In the end, he was merely having a tournament to test his strength and using the earth as a medium to get the Z-fighters to train and allow him to test his strength.
Frieza is by far the darkest villain, with only Vegeta competing. He tortured and murdered children not because he was told to, nor needed to. He wanted to, all according to his own will. The difference between Frieza and every other major villain is that he chose to be evil and has arguably done the worst things in the series. Whereas the others did what they were made to, and had honor to some degree.