for me it is absolutely Borg. if you ask why you can defeat Romulan, Klingon or other species and victory is yours. However, if you destroy Borg absolutely they will send much more ship to assimilate you. Secondly, you can't negotiate with them they just want you to come with them sector 001. Lastly, when they encounter with you they say "Lower your shields and surrender your vessels...." to dominate mentally and i think it works well. I think that and what do you think Trekkers?
I agree with you about the Borg. I find their "collective voice" creepy, almost demonic. Though the Vidiians scare me the most. Aliens that want to harvest your organs? Yikes.
Humans. We're unpredictable, members of our species are often shown engaging in bad behavior even when we have every advantage, freedom, and material comfort - often even *knowing* that we are going against our own best interests, just out of spite or some other illogical and/or dishonorable motive. You can count on a Ferengi to try to make profit - at your expense if necessary. A Vulcan to be logical, a Klingon to at least pay lip service to keeping his honor, the Dominion to try to conquer you to serve their Founders' insecurities, and the Borg to try to assimilate you. But you never know what will come next from a human.
TNG era Borg are pretty terrifying. Voy era Borg, eh. Just make sure the first guy they assimilate has a computer virus in his brain or whatever that was in Child's Play and Endgame. Ancient Vulcans are pretty terrifying. When you think about it, the idea that modern Vulcans are constantly bombarded with the impulse to savagely murder me and are just really good at controlling it is terrifying too. Founders, also. They didn't do half the things they should be able to do in the show.
Another vote for humans They successfully conned every planet in the Federation into believing it was an equal partnership and then they successfully infected every quadrant Every species has struggled with them (even Q are worried by them) Eddington said it best "You know, in some ways, you're even worse than the Borg. At least they tell you about their plans for assimilation. You're more insidious, you assimilate people - and they don't even know it"
I would have to say The Borg. They can't be reasoned with and they think they're right. And, when they want something, they're pretty tenacious about getting it.
The Vidians were nicely creepy. And let's not forget the flying neural parasites from "Operation--Annihilate!" And I'm going to go out on a limb and predict that nobody is going to name the Kazon.
Humans. Hands Down. Triumphant and hux said it well. I will simply add one of my favorite quotes: "You (we) are capable of such beautiful dreams and such horrible nightmares... Carl Sagan, Contact
The dominion are the most evil. But maybe the Borg are the scariest to have as an opponent. Like just_me said, you can't negotiate with them.
I found that unconvincing as well. It was just a cheap way for the writers to add an "emotional" adieu between Kira and Odo. Why would Odo leave Kira after he repeatedly said that he loved her above all? in an alternate future he even killed eight thousand people just to give his alternate self a chance to be with her... That's not something you do just for someone you'll leave only a few months after having finally got her. Talk about discontinuity!
I will join the others in saying that Humanity's the one that scares me. Not so much because of what we're capable of, when we get to warring or fear-mongering, but what we do, also, when we have the very best intentions in mind ... Most notably in TOS, but throughout the franchise, really, there have been civilisations viewed by the Good Ship Enterprise, or whatever, as being "stagnant" or "inHuman" and the STARFLEET people will just pull the plug on whatever's controlling it. Most often, it's a computer. Even if that computer's KNOWN to be in control by that particular culture and they take great pains to keep it well-maintained and fully functional, so what? Kirk or Picard or somebody's put off by it and they put a stop to it, then and there. Usually leaving the poor bastards to pick up the peices, while the ENTERPRISE warps out of the system, so it doesn't have to monitor the fallout and reconsider what they've done. Again, this is done with GOOD in mind! At least you expect Klingons and Cardassians and the Borg to be pricks. It's not surprising that they invariably are. But Humanity's supposed to be the best that a species can ever hope to measure up to ...
Humans are like insidious, Borg Root Beer. Much like the Borg, Humans/The Federation assimilate you without you realizing it. Much like root beer, you grow to accept it.
The Horta was scary when I was a kid. As an adult, it just looks like someone spilled lasagne on a rug. +1 on the flying creatures from Operation Annihilate. The Borg were creepy, but weren't scary until FC when they were mildly revamped. The subspace creatures from schisms were almost something out of a Whitley Strieber novel. A devolved Worf from Genesis was pretty creepy. Cardassians weren't scary until Empok Nor. Psycho Garrek was pretty spooky. Kind of a recall of the Scorpio Killer. Zombie Vulcans from Impulse were pretty freaky.