Who are the best villains of science fiction television? Though we found several of these choices to be crystal clear, some were a bit harder. We started thinking about the baddies in “The Prisoner,” “Nowhere Man,” “Deep Space Nine,” and we couldn’t decide. We considered individuals, like the Master from “Doctor Who,” Baltar of “Battlestar Galactica” and a certain young wife from “Firefly” who took down practically the entire crew. We thought about the almost-great villains, like “Star Trek: Enterprise”’s Xindi, and the ones that were sometimes villains but not always—like “Buffy the Vampire Slayer”’s Spike. We thought about "Lost," except that we don't even know who the enemy is yet. But finally, we had a top five list. And here are our choices…who are yours?

1. The Daleks of “Doctor Who.” The doctor, in all his incarnations, battled these evil machines for decades. Okay, their bulky shells couldn’t go up stairs until about the mid-‘80s, and their vocabulary mainly consisted of the words “Exterminate!” and “I obey.” But come on. They were great fun if only because of their single-minded quirkiness (humans are inferior and must die). Over the years we discovered many things about the Daleks—we met their creator, Davros, we encountered the Kaleds that were the original inhabitants of the metal casing and we watched them develop new colors, designs and abilities.

Every so often a Dalek reference will, even today, appear in a magazine like NME or in a TV show like “The Simpsons,” and there’s no bigger thrill. Plus, the sight of thousands of Daleks suspended in the air was one of the most impressive things of the first season of the new “Doctor Who.” There was a Hugh-like moment in the new series (see our statement about the Borg), but the return of these metal beasties was most welcome—and we can’t wait to see more of them.

2. The Shadows of “Babylon 5.” This epic series featured lots of interesting villains, although bad guys didn’t always stay so in this complex and dynamic universe. But the Shadows exerted a sinister, invisible influence on the B5 universe before they were ever seen. Who can forget the moment when the crew first viewed these cloaked, spidery figures on screen during the interrogation of the spy Morden? This mysterious race was one of the older races, like the Vorlons. But in shepherding the younger races along with the First Ones, the Vorlons wanted growth to come from order—the Shadows wanted evolution out of chaos. So there was war.

The Shadows awoke on their homeworld, Z’ha’dum, when an Earth Alliance ship landed there. Once awake, they started to build the forces they’d hid across the galaxy, offering the Centauri assistance in subduing the Narns without revealing their identity. The Centauri, buoyed by success, go on to attack other worlds. Eventually Captain Sheridan realizes that telepaths can confuse the Shadow ships, and an army of light, consisting of Narns, Rangers, First Ones, Mimbari and the League of Non-Aligned Worlds, is formed. But the Vorlons start to destroy any world that has harbored Shadows, and the Shadows retaliate. The conflict doesn’t end until Sheridan tells them that the younger worlds no longer need shepherding and won’t be pawns any longer. Rejected by the younger races, the remaining First Ones, the Shadows and the Vorlon depart to the Rim.

3. The Borg of “Star Trek: The Next Generation.” Not only did these part humanoid, part cybernetic creatures give rise to some of the best episodes of the series, like the two-part “The Best of Both Worlds,” they were genuinely frightening. They adapted any time they experienced phaser fire—they told us that “resistance is futile”—they kidnapped Captain Picard and turned into Locutus—and they just kept coming and coming. The Borg were an inexorable and destructive enemy. And the Borg brought out both the best and the worst of humanity, as all good enemies do. You had Federation veterans who would do anything to stop them, and you had lots of great dramatic material as a result.

In “I, Borg,” “Star Trek” diluted the Borgs’ menacing personality a bit by introducing a “virus” that helped form a renegade group of individualized Borg—thanks to a Borg named Hugh. Then the producers repaired that snafu in a feature film. But the Borg would be seen again in “Star Trek: Voyager,” in the character of Seven of Nine. The Borg have come to be another compelling part of the vibrant Trek universe, and we can’t imagine life without them. In fact, in popular culture today you can hear any unstoppable force (like Microsoft, for example) being referred to as “of Borg.”

4. The government in “The X-Files.” What can be more sinister than a government—our government—that’s trying to cover up the truth? Mulder and Scully’s constant battle to find out what our government is hiding is one of the most compelling fictional stories of recent years. Of course, we got occasional tantalizing glimpses of who might be involved, who was being manipulated, and how we might fight the power, but “The X-Files” managed to obscure the truth for nine stunning seasons. We watched as the government’s knowledge of alien abductions continued to revealed, as alien technology gave rise to supernatural terrors, as Mulder and Scully continued to be stymied by their own supposed allies, as informants came forward and got killed. It was the government’s actions that taught us to trust no one. It was great television.

5. The Cylons of “Battlestar Galactica.” From our modern vantage point, the Cylons of 1979’s “Battlestar Galactica” seem a bit silly—influenced by the disco era, no doubt, with their gleaming metallic armor and that moving red eye. There was Imperious Leader, with his silver robe and glass skill, filled with tube lights. You may also recall Lucifer, also a glass-and-fabric creation. But back then, we were very impressed and intimidated by these cool-looking robots, who were able to destroy an entire civilization using treachery and deceit, yet in retrospect were about as wooden and stiff as robots could be.

But it’s actually the Cylons of today that won these machines a berth in our top five. The Cylons of SciFi Channel’s “re-imagined” “Battlestar Galactica” aren’t just metal, although there are some Cylons that couldn’t be mistaken for anything but that. However, the creepiness of having Cylons in the humans’ midst—pretending to be friends, lovers, co-workers—is what has us tuning into this show every week. These Cylons have religion. They bleed. They get pregnant, for heaven’s sake. Unlike the Cylons of yesteryear, these Cylons are complex emotionally and physically—and they inspire reactions from the humans that are just as complex.