I find Umbridge the best villain in the series - although Bellatrix has her moments, too. But Bellatrix' blatant bloodlust is somewhat less scary. In my mind, Dolores enjoys hurting people, but she truly does believe that she does it for the greater good, her political and personal motives intertwined to form a web of self-righteousness.
Arabella is a very versatile character, I feel. We have been given just enough information to make her anything we want - from the dotty cat lady who does some low-level spying for the Order, to a formidable member of an underground movement who only puts on the facade of a dotty cat lady. And the cats are a good device to present her internal monologue.
Before you pointed it out, I've never realised how very much the structure does resemble that of a horror film. Dolores certainly would make an efficient monster.