I won't bother to get into the context of why we were talking about it, but I was asked Sunday evening: I don't think there's any question regarding your favorite hero, but who is your favorite villain?
I'm going to assume that what was meant was Comic Book Villain.
If you believe movie critics, superheroes are only as good as their villains. Now, anyone who actually reads superhero comics knows that statement is suspect, and should really read: a superhero is only as interesting as the challenge put to them. And in the movies, superheroes tend to run into their more colorful villains, usually pared down for a 2-hour action adventure, and cast with colorful actors who get to have fun with it.
Because the DCU and Marvel U are somewhat unending, and villains recur, its not the same, neat package one gets in a movie. Believe it or not, the occasional story doesn't even feature a villain (it's true!). So my criteria may be a bit different from what one sees in the movies.
It's also worth noting that superhero comics are respectively full of a multitude of villains for every hero (after all, Batman can't fight the Joker every issue, and Superman can't try to arrest Luthor every issue). Some appear once or twice, and then disappear. Others appear as an afterthought, and through the magic of re-use and popularity with fans, they can become as central to a company's entire line of comics as any superhero (ex: Lex Luthor).
There are various criteria one can use for a good villain. Costume. Motivation. General characterization. Initial vision for the the villain. How that vision has expanded over the years. And some may like villains for their somehow noble nature, such as Magneto or the occasional turn by Dr. Doom.
DC is littered with great villains, and I think the past two decades have seen DC really embrace and understand how to turn a villain from a cardboard cut-out to a three-dimensional character, all on their own. In a lot of ways, Marvel has been ahead of DC in this area since their inception. But, lately... Perhaps its pop-psychology, but villains now have rich pasts, goals, etc... just like their heroic counterparts. Hell, I'm a big fan of Geoff Johns' Flash thanks to his treatment of some of DC's second-string villains.
As classic as his heroes have become, I'm also a huge fan of several of Kirby's villains (which include several Marvel staples, from Mole-Man to Galactus. From Magneto to Doom). There's vision there from Kirby, and while it may not have always shone as brightly at DC as at marvel when it came to establishing a line of villains, he did some amazing work.
At the end of the day, my favorite villain (often copied, but never equaled) is Kirby's creation, Darkseid.
You might actually remember Darkseid from the 1980's version of Superfriends, "Galactic Guardians", which took an interdimensional/ interplanetary despot and made him safe for the kids. I assure you, he's not all about trying to make Wonder Woman his ladyfriend.
Darkseid's single desire: attain The Anti-Life Equation, which, once mastered, would give Darkseid complete dominion over all life.
If that doesn't give you a pretty good challenge for your resident heroes, I don't know what will.
Already without the Anti-Life Equation, Darkseid is massively powerful, perhaps more so than Superman, with whom he has gone toe-to-toe. He's obtained the power of the Omega Force (which he stole from his brother, seemingly killing him). He poisoned his own mother to become master of the planet Apokolips. He keeps several fiefdoms of power, constantly struggling for his favor, and the citizens of his planet are known as The Hunger Dogs, and are treated as nothing more than expendable slave labor to tend the fire pits of Apokolips, which fuel his engines of destruction.
With all that going for him, Green Goblin doesn't seem like that big of a deal.
Darkseid's intervention on Earth began in, of all places, the pages of Jimmy Olsen, when Kirby took over the title in the 1970's. It was part of Kirby's four-sided storyline of the Fourth World, featuring a pantheon of celestial beings who had brought their conflict to our backyard. From shadowy figure to fully realized despot, Darkseid would repaint the landscape of the DCU and provide the villain of villains, the final threat which Earth would always have to keep in the back of its mind.
Wonder Woman and Superman are two of his least favorite Earthlings, and there's significant history with other heroes as well. This is not to mention the ongoing feud with the peaceful counterpart to Apokolips, New Genesis, where his son, Orion, was raised and turned into a weapon against Apokolips. In fact, a treaty with new Genesis (mutually beneficial, lest the two planets destroy one another), is kept intact only if Darkseid never shows outward aggression toward Earth.
Darkseid, by the way, believes that portions and/ or all of the Anti-Life Equation exists within the human genome. And he's tried to harvest many times before.
Darkseid is infrequently hands-on, and prefers to allow his various minions, from Kanto the Assassin, to Granny Goodness, do his bidding. He also has a sychophant sidekick in the malicious DeSaad, who takes pleasure in pain (Darkseid is simply unmoved by suffering). When Darkseid finally does get involved, the earth tends to shake, walls crumble, etc...
In addition to the comics and Superfriends, Darkseid has appeared in the Superman and Justice League cartoons. Rumors also circle as to whether Lucas drew some of his inspiration for Darth Vader and the Death Star from Darkseid and Apokolips. That's a judgment call, but there are certainly similarities.
What Darkseid would actually do with the Anti-Life Equation is unknown. Unlike Mongul, his cheese-colored counterpart, Darkseid is not about suffering. He's far more about control, and would likely squeeze free will from every last being within his reach.
As a "god" (little "g"), Darkseid has time on his hands, and so he's as much a schemer and planner as anything. He plays games for power with beings like Eclipso and Brainiac, and thinks little of forming alliances with Earth heroes when the occasion calls for it (Our Worlds at War).
In short, he's a big, spooky dude. Who can also atomize you with the red Omega Beams which fire from his eyes. You can't distract him to save yourself, as he is single minded about the Anti-Life Equation. You can't offer him anything he doesn't have as ruler of a planet, but the knowledge of the Anti-Life Equation and what might lie beyond the Source Wall. While not containing the flawed nobility of a Magneto, Darkseid is almost difficult to label as twisted and bent like so many other heroes. Instead, he's a single-minded force of destruction, like a force of nature in the DCU.
In many ways, of all the villains in the DCU, none has carved out their own mythology to such a degree without relying on a co-dependent relationship with a superhero to define them. That's a respect the writers and artists seem willing to give Darkseid, recognizing the place in the DC pantheon which Kirby created for him. And I've always found that more fascinating than, say... Sportsmaster.
And, yes, Thanos is interesting, but he's sort of the poor man's Darkseid.
Hope that answers your question.
I'm going to spotlight a few more villains over the next few weeks. This has been kind of fun.