Well this is a campaign debate, so allow me:

my #1 pick: Goddess Vicius

I've only read up to and including volume 10 so I'm not up to date, but I found Viscious always to be very 1-dimensional evil. Like her entire personality is that she's evil but beautiful. She was so far only reactionary without any godly insight or forethought and for being a goddess she's honestly not very bright. She gets completely blindsided when someone who has all the reason in the world to hate her schemes against her and doesn't seem to have a personal motivation for what she does other than to fulfill her daily evil quota. Honestly if in one of the volumes later on it would turn out that she isn't a goddess at all and just a lazy otaku neet who only got reincarnated herself it would be more believable than trying to sell her as an all-powerful being. She is like Walter White trying to look all cool and in control but in reality is scared shitless and swept away by what's happening around her, desperately treading water to keep her from drowning. She is a caricature and a joke.

No, compare this with my, clearly far superior pick! I present you my candidate (cough actual former God chough) Erwärmen!. Not only has he a complete backstory that can fill an entire book itself, but his motivations are clearly defined. He isn't the antagonist because MC needs someone evil to fight against, he is the antagonist because his goals are reasonable and just but ultimately clash with the personal wants of Rozemyne. Like every great SSR-Tier antagonist before him (Prince Nuada and General Hummel) he is the hero of the story if you only shift the POV by a fraction of a degree.

That's, Bookworm and .. Lady Bookworm is what an Antagonist of the Year looks like!

I rest my case.

ps.: Fran & Teacher is a good pick. Granted, not Clarissa & Hartmut levels of good, but respectable nonetheless.