Everybody loves a "good" villain. Not one who is actually good, but a well-developed and well-rounded villain that causes so many problems for the hero to overcome.
The more problems the villain causes, the more interesting the story becomes. And I don't mean those 'evil' villains that you can see coming a mile away dressed completely in black and stroking a cat while bragging about his latest scheme to take over the world, I'm talking about the subtle villains who keeps the reader guessing as to their real motivation.
These are the ones I love to read, and also writing. I have a tendency to try and hide the real villain of the story to add a twist and make the stakes at the end higher than if the villain's known.
Another villain I love is the one who believes they're doing things for the greater good, even if that includes killing many dozen innocent victims in the process.
What sort of villains do you like? Do you prefer to keep guessing or do you like the James Bond style villains?