Okay, so now we're all up to speed. The thing about this is that it's true! I hate Umbridge so much more than I do Voldemort. And I'm willing to bet that's the same for you. (And if that's not the case, than you're just a tad crazy.) Anyway, sure, it's been years since I first saw this, but I've recently been really thinking about it. What makes those truly nasty, horrible, despicable characters such good villains?
Picking on the main character's weaknesses
Some villains are just out there, doing what they're doing for "the greater good". The greater good doesn't necessarily involve the main character all that much as an individual, but when the villain is always there, attacking the main character's insecurities and weaknesses, you have to loathe them automatically.
Another thing is simply when the villain has all the power, and they can easily get away with everything they're doing. Take Umbridge for example. Every last cruel act and horrible rule she got away with because she controlled the law. With the law on her side, there was literally no way for her to get in trouble for what she was doing at Hogwarts. That feeling of powerlessness and despair, when it seems like there's no way out, makes it very easy to hate the villain.
Being tangible
Another concept I find makes a stronger villain is simply when they're tangible. A luminous villain who lurks around in the back burner of the plot as more of a "concept" than a real being is difficult to actually loathe. Sure, they provide a conflict, but I don't hate them. Which, in some stories, that's perfectly fine, but for many others it's not.
No redemption
Some villains in some stories turn good in the end. They see the truth behind their ways, and decide to change. Sometimes, villains have redeeming qualities. Whether they had a messed up childhood, a lost loved one, or some other traumatic experience, there are some cases where you can somewhat sympathize for the antagonist. However, when the villain is purely evil- they simply glow in the fire from the pain of others, it's so much easier to hate them. When a villain's views are purely evil, when they have no moral ethics, and when they just don't seem to care about other people's happiness, boy do I love to loathe them. (At the same time, a complex villain does seem to make for a better character, I just find it easier to sympathize for them.)
Able to relate to the main character
This is directed more towards the realistic fiction genre, although there are certainly some exceptions, but when an antagonist treats the protagonist in a way you can relate to, I feel like I can dislike the antagonist more. I'm talking about simply being mean, bratty, jealous, or unlike a true friend. When you see a rendition of someone you knew (and weren't particularly fond of) come to life in a book, it's easier to dislike them.
Love to hate
So what am I saying makes a good villain in my opinion? Someone who isn't just a generic bad guy doing this for the "greater good". When a villain is targeting the protagonist, attacking their weaknesses and insecurities, and taking everything the hero loves most about their world away from them, they end up as a villain we all love to hate. When the villain is a tangible person, someone who could really exist in this world, and someone we actually see occasionally, you get to know them on a level fit for strong dislike. And when the villain has no hope for redemption- everything they do is out of spite, hate for humanity, or personal gain, there just isn't any sympathizing.
So what do you think makes for a good villain?