A race so alien that the producers eventually ruined them by making them more familiar and 'comfortable'. When originally presented, the Borg had a genuine hivemind -- the entire collective is the reacting organism, there is no central control at all. Creatures that can be considered as having hiveminds, like ants or bees, don't have any special organizing hierarchy. The queens don't rule, they're just the source of new members. If the chemical signals they send or respond to are tampered with, the other insects will rip the queen apart. As awesome as Alice Krige's portrayal of the Borg Queen was, the character was a bad idea.
They DO remember. They’re essentially trapped in their own bodies beating at the inside of their own skulls as frozen spectators of their former bodies movements. A living representation of “I have no mouth and I must scream”. Terrifying.
6:35 the Borg target Earth because the Enterprise "snapping" across half the galaxy in the blink of an eye is a feat they've never seen before and acquiring that technology becomes a priority. They are unaware it's the omnipotence of Q that enabled the feat. They change behaviorally between appearances because their opinion of Humans changes.
What's frightening about the Borg is that they don't kill you, they strip you of your autonomy, render you powerless. There was always the sense that a part of your awareness or consciousness was trapped inside what they remade you into. Like a human in a vegetative state who nay or nay not experience some awareness. That kind of helplessness is awful to think about.
The Borg started their downward trajectory with the introduction of the Queen, an element forced into the narrative by the executives who wanted a clearly defined individual 'bad guy' for the characters to deal with - which is the very antithesis of what the Borg are as originally presented. This was then seriously compromised further by Voyager overusing. That's not to say Voyager didn't do some great Borg themed stories (and we got one of the best characters out of it too - Seven of Nine), but when Voyager is casually taking on not just a regular Borg Cube but a freaking Tactical Borg Cube and surviving just fine... yeah no, that's not a scary villain anymore.
I think Voyager did 'Borg are scary' best when they had 15 Cubes scream past them and Chakotay just goes "My god". That was awesome.
I don't quite like how the Borg Queen was handled in First Contact, but I do like the idea of it if done a spcecific way. Imagine you find out that the Borg has a queen, and the federation spends a lot of time trying to figure out how to exploit the queens as a form of weakness, only for them to finally destroy a borg queen, and it does.... nothing. Because borg queens were just red herrings, and tricked the federation into pursuing a weakness that didn't exist. Kind of like how in Terminator Salvation, Skynet had a code that disabled their terminators when exposed to it. But then it was revealed that it didn't actually work, it was just a ploy to get the resistance to transmit a powerful signal for synet to trace, because it tricked the resistance into broadcasting the code that didn't actually work.
Episode 4x02, the one immediate after The Best of Both Worlds, where Picard goes back to France, is easily my favorite Jean Luc episode. When he breaks down to Robert and cries, "they took everything i was" and his brother replies, "So. My brother is human being after all." so powerful. Shows us that Picard made the transition to hell and back. Wonderful story telling.
What I quite like about them in the earlier episodes is that even though they are a giant horde of faceless drones, they are not cannon fodder. Any other show would have our heroes blast them away left and right, but the way it was executed, even a single one of them posed an immense threat.
In a way the comparison of the Borg when we first see them to how they are portrayed in later years is similar to the Weeping Angels in Doctor Who. The more the writers explored who they are, the less effective they became as antagonists.
The phrase "Resistance is futile", spoken by Picard after he is assimilated by the Borg, gave birth to an emotion not yet experienced by kids like me watching that episode.... Dread.
That first episode in that two part series was, in my opinion, the best script ever written, and not just for TV.
The interesting part is that it was written as a way for Patrick Stewart to exit the franchise as his contract was up and he didn't want to return for another season unless they changed his character to be more serious and thoughtful. So if negotiations failed they could just end his character in the second part.