My, this is a tough one. They all have their plusses in the the witchcraft field, but set-backs as well.
SPOILERS:
Angelique- If I remember correctly, Angelique was originally made a witch by Judah Zachary in 1692. She said that he had almost "tricked" her into joining his coven. (using the same techniques he was using with Daphne) She could make a good witch, but her spells are often very rash and she ends up regretting them. (Ex: the vampire curse). Plus, I always got the impression that she wouldn't even use her magic much if she wasn't always chasing after Barnabas. Barnabas could probably have so spared himself so much trouble if he realized he loved her much earlier in the series! She has some good spells, but she is too often blinded by her love.
Nicholas- He was a wonderful warlock before he got involved with Maggie. He had everything down- the evil grin, the gray ensemble, the hand motion when he places spells (I love that!). He seems to be pure evil. He loves torturing people (especially Angelique) and is very conceited. All in all, an excellent warlock.
Judah Zachary- He was quite the evil man. His sneer was the worst. He never cared about anyone, only himself. As some have said, Angelique truly believed she was right in doing what she did, while Judah was just plain evil. Plus, he just always seemed the most evil to me.
Overall, I think Judah wins the award for the most evil. But, Nicholas wins the award for the best sorcerer. Angelique has probably the most human emotions of all of them.
-Lindsey