I adore villains. They are almost always my favorite characters in books and movies. I don't know why, but there is something about a bad guy that makes my heart flutter. And even a few bad girls can do it for me too.
The best villains are three dimensional, real people who have been forced into the lifestyle they lead. They are not inherently evil, they just do what their set of morals think they can do. And the hero/heroine lives by a different code of ethics.
I always tell people that the mice think the cat is evil, but he's not. He's doing what he feels is right. And good villains are this way too.
Of course there are antagonists that are completely evil. They are selfish, heartless, and driven to get what they want. These can be fun too...in small doses. I like the mean, spiteful bad guy once in a while, but I really do like to feel for my villain. Sure, I want the hero/heroine to prevail in the end, but it makes for more interesting reading to see the progression from just a guy to holy crap this dude is a jerk.
My villains tend to be, well, not really people. I have bad guys in some of my books, but in the majority of them the antagonists are the characters themselves. What? Yes, it's true.
Their feelings, their emotions, their own wants and needs are getting in the way of their own happiness in most of my books. These stories don't need a person to screw things up because my characters are doing that just fine all by themselves.