Difficult question. Sympathetic villains are out, since the whole point is that they're not evil fundamentally and are just made that way by their experiences. But demon characters that symbolically represent evil itself don't feel like the right answer either, not just because they're too obvious, but because they're usually written to just be fundamentally evil, by nature and not by choice. You can't really blame a demon for torturing people.

I think the my favorite category of villains is those whose are unambiguously, purely evil, but still have a compelling explanation as to why they are evil and they how got this way, as a cautionary tale to avoid becoming like them. Star Wars: Andor does this really well with the Galactic Empire, but part of why it's great is that no individual character fully embodies the evil of the Galactic Empire. (None of the Sith Lords ever show up.) They're all manipulating each other for their own ends, or acting out of loyalty to the system.

So my vote goes to Gul Dukat from Star Trek: Deep Space Nine, who also fits into the category of "pure evil, but for a reason". He's a fascist leader directly involved in a genocide, desperate to restore his power after his victims staged a revolution. The show is not subtle about his being evil; Captain Sisko even says that he didn't believe that pure evil existed until he saw Gul Dukat, and there's a Biblical allegory throughout the series in which Dukat stands in for the Anti-Christ. But he's also a complex character, whose evil actions make sense given his experiences and the system he grew up in. He's clearly going through stages of grief over his loss of power, yet never reaching acceptance, and never truly improving as a person.

Seriously, watch Andor and Deep Space Nine, they are SO GOOD