Science fiction has served readers and viewers aliens of all shapes and sizes over the years, from HG Wells' beaked & tenticled blood-sucking Martians to ET to the weird, rooster-lookin' supreme being at the end of Dan Simmons' Muse of Fire. Then, of course, you've got your various non-corporeals like Arthur C Clarke's monolith builders or Star Trek Deep Space Nine's wormhole aliens. But sometimes these beings take on human form. They're not to be confused with extraterrestrials who just naturally look like us, like Ford Prefect or Klaatu. These are creatures who look nothing like us but for various reasons have chosen to mimic our form. Here are the best among them.
The Top 5 Aliens in Human Form:
5) Mirror Girl - Blind Lake by Robert Charles Wilson
For years, all that any adult knew about Mirror Girl was that she was a figment of the imagination of a socially-awkward little girl living with her mother at a scientific installation. By the end of the book, they discover Mirror Girl is a very real alien entity - the gatekeeper of a very strange device that facilitation observation, communication, and possibly travel between different worlds. While mirror girl certainly isn't evil, she isn't bringing flowers and candy to humanity either. There's a creepiness to her interaction with the protagonists at the end, and especially with the way she uses the child Tessa to observe things over the course of the story.
4) Centauri - The Last Starfighter
This old con man/military recruiter isn't a shape-shifter and doesn't use telepathy to influence how others see him. For Centauri, image alteration is old school - he mashes a mask over his face and slips on a fake pair of eyes. Centauri is a character who sees the rules as being as flexible as external image, but ultimately, he's working for the greater good - although "it never hurts to be rich." He's certainly one of the most entertaining secondary characters in science fiction who dons a human face - and he's got a slick ride and knows how to rock a hat.
3) Ambassador Kosh - Babylon 5
The Vorlon ambassador is almost never seen outside his encounter suit in his natural form by anyone besides members of his own race and a few others. On one occasion he appeared to a group of diplomats on the station as representations of each of their cultures' versions of angelic beings, while rescuing Captain Sheridan from a potentially fatal fall. But "angels" don't count for the purposes of this list. What does count is the season 3 episode "Interludes and Examinations" where Kosh takes the form of Sheridan's father during a telepathic conversation/dream. It's a way for Kosh to put aside the zen koans and cryptic pronouncements that he usually communicates with, a way to show Sheridan a comforting, trustworthy face to have a straight-forward talk. Something entirely appropriate for Kosh to admit to Sheridan that the human is right and has done a good job, that the Vorlon ambassador was afraid, and that he's saying goodbye (sort of). It's an extremely poignent scene, cutting between Kosh with a human face saying farewell in Sheridan's dream, and Kosh in his natural form (which we, the audience, still don't really get a good look at), slugging it out in his quarters in a fight with a gang of Shadows that he's doomed to lose. Kosh only appears as a human once in the entire series (I don't really count the hand shot that Leeta recalls in the pilot), and it makes that appearance all the more powerful because it's done in this circumstance.
2) Starman - Starman
In this story, a not entirely non-corporeal alien (it appears as a ball of blue energy, and yet apparently has enough in the way of physical needs that it needs to get home/back aboard its ship in order to survive) uses its technology to clone a human body to inhabit so that it can go incognito in the US and survive long enough to make a cross-country to make a rendezvous with its mothership. Along the way, it experiences human sensations, interacts with the natives, and falls in love with the woman it's kidnapped (the widow of the human it cloned), and in so doing, it learns - at least a little - what it means to be human, conversely discovering a bit about its own species and what they're lacking. Starman is also worth watching to see Jeff Bridges' performance as the alien. You can see why Bridges was nominated for a Best Actor Oscar the year the film came out - he's extremely convincing as a character that has to learn how to use its body from the ground up, how to communicate with humans, and how to navigate American culture. He's totally believeable as alien - even at the end, when the character has experienced so much, developed a degree of understanding of it, and has enjoyed some of it, for all his appearance, this character is still undisputably inhuman.
1) Q - Star Trek the Next Generation/Deep Space Nine/Voyager
Was there ever any question about who would get the number one spot on this list? Whether he's posing real, lasting harm by forcing a collision with the Borg or threatening to extinguish humanity himself, or whether he's simply being a pain in the ass playing jokes on Picard or making fun of Riker, Q is the most memorable alien to have taken human form. Admittedly, that's a strong claim to make considering shape shifters and highly-evolved entities taking human form are par for the course in the Trek franchises. But John DeLancie's performance was so deliciously over-the-top that any time Q was on screen you couldn't take your eyes off him. Q may have been guilty of many things, but never of being boring. I think ultimately part of the attraction of Q's character is that for all his power and knowledge, the fact that he looked like us reminded the audience that some things never change - it was a visual statement that even if our species were to attain such evolutionary heights, even if we were to shed our corporeal forms and be able to look like whatever we wanted ("We've all been the dog!"), we'd still probably behave the same, we'd still look like humans on the inside.
So who/what are your nominations for the best aliens to take human form?