I was going to pick Aizen but like c'mon that's like picking Freiza, Madara, Zeref, or Doflamingo. So I'm picking a different antagonist :3
Lucia: Rave Master I like Lucia because he to me was the exact opposite of Haru. He wanted destruction for the sake of destruction to do so hg needed all the Mother DB's and during a fight against Haru he made Haru well beat the hell out of him. Lucia is a fantastic character who's power was unmatched by protagonist Haru and even with time Haru still couldn't beat him, Lucia is a great example of what an antagonist is supposed to be, menacing, powerful, and proper motivations.
You have any antagonist that you love? I would love to see your cards on the subject and check out the original post/collection from @SimplyAwkward