I favor villains of some complexity. Creating a monster is much too easy. And less engaging for a story.
I was binge-watching a TV series, and a villain showed up with lots of power, a clear flaw (greed), a philosophy that hinted at a sophisticated worldview, and evil henchmen. He also seemed to be pretty smart. Though he did not show up again in the episode after his introduction, I knew I had not seen the last of him. And I had hope.
Sure enough, he popped up a few episodes later in the season.
Great! Get the popcorn.
But all he turned out to be was a hero-torturing monster. Sigh. (Even worse, his evil henchmen were inept, and their security measures failed completely.)
The villain escaped. I don't care to see him again. And that is the problem with monsters in general and unimpressive ones in particular.
So... the first rule on a villain. Make him, her, or it (but not them) a worthy adversary. Even actual non-human monsters, can reach that level if there is real doubt about the hero's victory. This means providing a demonstration of huge physical power (King Kong), unpredictability (The Thing), a special talent/skill/power (almost any comic villain). Show a tough opponent being beaten.
Now, to go beyond this kind of a monster with humans, I think intelligence is necessary. It is too simple for a reader or an audience member to imagine a fool being beaten. This does not mean the villain can't hide his/her capabilities. Uriah Heep is all the more loathsome because he pretends to be humble and subservient. And it's always fun to have a master villain pretending to be a minion, putting his/her second out front as a shield or bait. Surprises are always welcome.
People are always looking for how the hero might triumph, and it is harder to triumph over a smart villain so don't loose tension by making the bad guy/gal a fool.
As I've written before, Damon Knight advised having a ratio of about 70/30 good to bad for protagonists and the opposite ratio for villains. I'll take that further on villains. It is pure gold to have a villain people can really hate. But the gold gets transformed into platinum when there is a piece of them readers or audience members love and connect with.
As with heroes, talent, humor, and having been wronged can help us to connect with a villain. I think there is also value to exposing doubt in a villain. Or compassion for their foes. Or past good done. Or one wrong turn that set them on an evil course. When I see myself in a villain, when I think, "There but for the grace of God go I," the appeal jumps. Darth Vader, not Godzilla.
Story-wise, making elements of the villain reflect elements of the hero enriches their conflict. It shows the duality of powerful human traits. And, if the hero sees him/herself in the villain, that brings everything up another notch. Then we have the character we are identifying with wondering about what's right and wrong, what's good or bad. And it's personal, leading to a necessary look inside and a reevaluation.
One more thought -- this on the "wrong" turn. Certainly, a promising character can become a villain because of a trauma. Have the most talented kid in the community first witness the deaths of family members and then be kidnapped, abused, a brought up in a crude and ruthless community, and you have a super villain. Loss, deprivation, isolation, and injustice to a vulnerable individual can turn out badly. Good, but perhaps too simple.
I think exploring corruption provides more of a payoff. There are amplifiers that reveal character. Think of celebrities and powerful people who have been caught taking a vice to its limits. Think especially of those who have touched our hearts or braved adversity or made us laugh or gained a victory at great personal sacrifice for human rights -- and then shocked and disappointed us. In almost every case they have been corrupted by amplifiers. Wealth. Power. Fame. Honors. The social gifts that delay, diminish, or destroy accountability.
Lead us not into temptation. Perhaps it means don't put me into a position that amplifies my weaknesses by making me unaccountable. Don't give me gifts that corrupt my character.
But, as a writer and a creator of villains (and, to an extent heroes), it may be your job to corrupt promising and outstanding characters. It seems evil, but, with villains such as these, you can develop exquisite human moments within you story.
Next time, I'll continue this exploration of villains with motivation, how to rouse them to extreme action.