*cracks knuckles* Alrighty, here we go!

I love writing antagonists. They are arguably the most important character you will ever write in your game/book/movie/what have you. Without a good opposing force, the protagonist has no reason to ever get better or improve. Plus, there's so little threat, all the characters feel flat to begin with.
Now, it also depends on the antagonist you want to make. If you want an evil brute that is scary and pure evil and there simply for people to hate and oppose, that's one thing. They will serve to be the villain and not the antagonist.
Let me put this simply - an antagonist is the protagonist of their own story. If you were to be playing THAT character instead of the protagonist/main party, you as the player would empathize with them, understand their goals, maybe agree with their goals.
That is scary enough in my opinion. When you begin to think, maybe the antagonist is right. When you start to sympathize with them.
But, if that's not scary enough for you, I also love the antagonist that is so far above the protagonist's level, they find the protag 'cute' or 'funny'. In the 'I am far stronger than you, and you still try to oppose me? Oh, that's adorable.' I'm not talking the tried-and-tired unwinnable battle (sorry...it is a tired trope...) but just a level of utter dismissal by the antag. The protag is so far beneath them, they're not worth the attention of the antag.
When an antag doesn't even take you seriously, you know you're small fry :3
To me, the scariest antagonist is one that is relatable, has reasons, one that may even be right in what they're doing. These are also great antagonists, as they could very well be protagonists if you were playing from their perspective.