Umbridge for sure. She was absolutely despicable and I felt zero sympathy for her. The others I can have sympathy for (some more than others). As mean-spirited and cold as Snape could be, he wasn't evil just a broken, lonely man. Quirrel was used and possessed by Voldemort, he was just a pawn in his game. However, he loses sympathy points for trying to kill an 11 year old boy. Mad-Eye was misunderstood and strange but he cared for Harry. Lupin, (I love him) has all my sympathy given his condition which must've been such a burden to bear but he still remained a kind-hearted person despite this, and was like a father figure to Harry. Gilderoy well... he was just an egotistical airhead, I'm indifferent towards his character.

Dolores Umbridge is the Pure Evil Monster.