It's no secret that actors love playing the villain. Many state that the villain is the most interesting character in the story, so they get to use most of their acting skills in delivering the performance. They're not wrong.
Of the two characters that are load-bearing pillars in narrative construction, the antagonist has to be consistently engaging and interesting while being repellent to all people of good character. If you don't have this in your antagonist, you have a simple deuteragonist instead. (This is not a bad thing; it merely requires that you take the time to reconsider your story's narrative and revise it to fit your goals.)
Put simply, a good antagonist is both charismatic and wrong. He has to be charismatic because he needs to capture the attention of the audience and hold it firmly- that's charisma. He has to be wrong--morally most of all--because otherwise why is the protagonist opposing him? (We'll get into Protagonist-v-Deuteragonist narratives another time; this is "Getting Started" not "Refining Your Craft")
Put a good deal of thought into your antagonist's plan. Most old-school adventure tales where an antagonist is the source of conflict revolves around the protagonist seeking to stop the antagonist from getting what he wants. The reason is that what the antagonist wants is wrong--immoral--somehow.
Go figure out what that is, and the odds of your narrative falling into place goes up dramatically. From that point, it's down to figuring out when and how the protagonist intervenes; that's your inciting incident, most of the time, and if that turns out to not work, then you can walk your story via revision to where that point actually is incrementally.
Then remember that your antagonist needs to be present throughout the story's narrative, literally or by proxy, so that your protagonist has good reason to be the hero in your story. Even when neither Vader nor Palpatine are on the scene, their presence is felt in the Original Trilogy. Even when the protagonist has to deal with secondary villains, that presence is felt. ("We've got deal with this now so we can get back to dealing with the real threat!")
There's plenty more, and more specific, advice on just that sort of thing around. We're not a shy bunch here, so I am certain that those of my friends and allies who already have articles and posts on writing an entertaining and believable antagonist will post the links in the comments below for your benefit. In the meantime, go review your favorite antagonists and see how they work in their stories; you can't go wrong by stealing what works and making it your own.