I don't know if they're my favourite villains, but here are some I think are pretty well written/ones I absolutely despise the most:
Fukiko Ichinomiya (Oniisama e)
Griffith (Berserk)
Bondrewd (Made in Abyss)
Funny Valentine (JoJo's Bizarre Adventure Part 7: Steel Ball Run)
And here are some dishonorable mentions:
Mahito (Jujutsu Kaisen)
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