Adam Taurus. Could have been the best villain in RWBY if they actually showed us the oppression Faunus go through and didn't make him an obsessive ex in Volumes 4 and later.

Not completely wasted, but REALLY half-baked - Kenjaku from Jujujtsu Kaisen. I relly like the concept of mad scientist who lived for so long that to satisfy his scientific curiosity he's ready to doom all world. My main problem with him it's that plot doesn't really need him after start of culling games. Below there's things that could make him better character.

1) Just give him a time to opposite Yuji like with Mahito and Sukuna. Otherwise, it's just strange that despite being Yuji's creator, they barely interact with each other. It's like Luke would never know what Vader is his father.

2) Explain a little bit more why he became like that (not necessarily give him tragic backstory or something - just make him tell, for example, what he doesn't care about people and world as whole because he lived for too long time).

3) Also give him idealogical foil. Mahito has Yuji. Sukuna has Gojo (and Yuji). Kenjaku doesn't really have something like that.

Because of lack anything i described - he feels more like plot-device rather than character

The question is good, but unfortunately the list would be very long.

makoto kagutsuchi would've been better if he wasn't an anti-villain and i think kodaka doing that was a horrible horrible idea (him being an anti-villain is why he's repeatedly downplayed by the fandom that knows him)

I personally would've liked Flowey from Undertale better if he were treated as a straight-up, inexcusably evil bastard who can't be redeemed instead of just being another "oh he's actually a nice guy if you look past the atrocities he's committing and he has such a sad backstory, pls feel bad for him" like most of the game's antagonists. Basically if he had nothing to do with Asriel and is just a plant that was given sentience and the power to control time, which made him see himself as separate from everyone else and naturally treat reality as a game since the suffering, death and happiness of others have no real value when all of it can be undone. That would've made him stand out more, added extra nuance to the game's narrative and made him a more fitting mirror image to the player.

Screenslaver. Had the potential to be great but her plan was stupid and she didn't have enough screen time.

@Unremarker but the whole point of the game is that things shouldn't be solved with violence

@The nameless master Yeah but in real life there are tons of people who are just off the deep end one way or another and you can't expect to be able to reason with them, which the game doesn't really have proper representation for in my eyes. There's a lot of other things I could go on all day about but probably shouldn't here.

@Mich Na204 Yeah if I'd rewrite JJK, I'd do away with Kenjaku, he doesn't feel like a real character. A 3 way war between Geto and his cult, the disaster curses and the jujutsu sorcerers with Sukuna as the overarching threat (and final boss) would have been a better plot imo

@Unremarker are you an undertale hater?