Everyone has their favorite villains. They are the people who help make the hero, they raise the stakes and they also usually have the best lines. They are the cause for the conflict and without them...well, there wouldn't really be much of a story, would there? They help us to define the notion of good vs. evil (though there are a lot of grey-area villains).
Each and every villain has that one thing that we all love about them. For me, my favorite villain is The Joker from The Dark Knight. What I loved the most is how he managed to bring out the worst in the three men opposing him: He drove Harvey Dent to total insanity and made him a killer. After that, Batman and Jim Gordon were so afraid of the truth of Harvey's action coming out and its impact on Gotham that they decided to name Batman as the killer instead. Because they didn't believe in Gotham being able to handle the truth. The worst part? They were proven right in the third film, when Bane revealed the truth. None of this would have happened without the Joker manipulating events. What an impact he had.