Considering "favorite villain" would mean the person we hate the most, I will agree with redhotght. Umbridge just got on my nerves, and I just felt like slapping her a 100 times and feeding her to Buckbeak every time she entered the scene!
She was just so pure evil
. I mean, Bellatrix (Black) Lestrange being addicted to Dark Arts was expected, as she grew up with the Dark Arts, breathed the Dark Arts, and lived
the Dark Arts since an infant, but we don't really know the history of Umbridge, so we can't really understand her cruel twisted mind.
But, considering that "favorite villain" would mean the person we hate but also are fascinated with, I would definitely choose Bellatrix. Man, I hated how she killed poor Sirius, but I absolutely looooooooooooved how Molly Weasley killed her! To think, Molly Weasley of all people killing Big Bad Bella!
Harry Potter and the Deathly Hallows: page 736
He changed course, running at Bellatrix rather than Voldemort, but before he had gone a few steps he was knocked sideways.
"NOT MY DAUGHTER, YOU BITCH!"
Mrs. Weasley threw of her cloak as she ran, freeing her arms. Bellatrix spun on the spot, roaring with laughter at the sight of her new challenger.
"OUT OF MY WAY!" shouted Mrs. Weasley to the three girls, and with a swipe of her wand she began to duel.
~~~~a few sentences later~~~~
"You--will--never--touch--our--children--again!" screamed Mrs. Weasley.
Bellatrix laughed, the same exhilarated laugh her cousin Sirius had given as he toppled backward through the veil, and suddenly Harry knew what was going to happen before it did.
Molly's curse soared beneath Bellatrix's outstretched arm and hit her squarely in the chest, directly over her heart.
Bellatrix's gloating smile froze, her eyes seemed to bulge: For the tiniest space of time she knew what had happened, and then she toppled, and the watching crowd roared, and Voldemort screamed.
LOVED this scene!