This depends on your goals and desires for the emotional trip that a reader may go through in your story. Take a look at Game of Thrones. We have characters like Ramsay and Joffrey who were antagonists. They were good though because they were someone you can hate. They were doing many bad things to good people. Everything about them made you want to punch your T.V. in hopes that you could hit them too.
Then we have stories like The Martian or Castaway where the antagonist is not a person at all but the environment, time, nature, their own self and not letting despair take over.
It all depends on how you want the story to be. Do you want to make the antagonist evil? maybe the antagonist wants to take over earth but is doing it out of necessity with no other choice and is actually a really nice being. If the story was told from his POV, he would be the protagonist and the earthlings would be the antagonists who are getting in his way of saving his race.
It really does boil down to how you want the story to flow and how you want the "bad" guy to be presented.
A quick google came up with this link that may help you refine your needs of how to write a better antagonist. There were several links that came up with an easy google search: How to create a good antagonist.