I'm actually a bit blown away by the poll result; the good old illithid was my pick for victor but I'd expected the drow to be running a close second, lavender eyes flashing in the darkness. Possibly it's time for Bob Salvatore to start up a new series about a good-hearted mind flayer living in self-imposed exile, tortured by the brain-eating ways of his kin?
I'm a bit baffled by the two guys who voted for Derro (or possibly one guy on two computers). Really? Derro? I'd love to hear a compelling argument for their awesomeness; I've always considered them the also-rans in the evil dwarf marathon. I guess this is why we have polls; to establish that Kuo Toa are scientifically three times as popular as Derro, despite apparently
But let's look at why the mind flayer wins.
Mind flayer and illithid are interchangeable terms; I'm not sure if they're used in connection with each other as far back as the mind flayer's original appearance in 1975 but certainly by the late 80s and the birth of AD&D 2nd Edition they're inseparably pinned together.
Mind flayers are notable for their humanoid appearance, topped by a squidlike head festooned with writhing tentacles. These tentacles burrow into the brain of the mind flayer's victim to feast on the goo inside. They are deeply intelligent, often displaying genius-level cognition, and are naturally psionic, able to deploy a devastating mind blast from which they partly derive their name. In addition, many illithids are accomplished magic-users and they make use of cutting edge magicpunk-style technology (culminating in the titular extraplanar vessels of the Spelljammer campaign setting).
I think the reason the mind flayer has become so iconic is the way it straddles so many horror and fantasy tropes so effectively. Not only is an individual mind flayer a mastermind-level campaign villain, it comes from an entire civilisation of such beings. It's a deadly psion, able to turn your own thoughts against you, and at the same time it's a powerful archmage (some mind flayers ascend to undeath to become a ridiculously-named illithilich.) Its humanoid appearance lets it fit any number of regular character roles, but at the same time it's a Lovecraftian abomination, a Thing-That-Was-Not-Meant-To-Be. 4th Edition has mind flayers once again hailing from the Far Realm, which is in keeping with their whole Elder Gods flavour.
There's also the ongoing theme of penetration and invasion. The mind flayer is one of the horror genre's rape-surrogates. The illithid invades your thoughts; its tentacles drill into your brain; and its spawn, the repellant illithid larvae, grow to adulthood by entering a parasitic and ultimately lethal relationship with a living human host.
Mind flayers are a huge number of successful horror monsters rolled into one; and if the basic illithid doesn't completely meet your antagonist needs you can graft an illithid larvae onto any number of other monsters to create a ridiculous mind flayer hybrid - anything from the half-svirfneblin "mozgriken" to the beholder-illithid "mindwitness" through to the too-awesome-to-be-silly "brainstealer dragon".
Personally the image that stays with me of the mind flayer is of an illithid, its arms folded within its voluminous robes. It doesn't need to unfold its arms to kill you. It doesn't need to move. It controls your brain, and you walk towards it, and it feeds.
Man, I love those guys.