The Borg are the most badass evil alien menace ever to grace science-fiction. They make the Empire look like a bunch of pussies.
Sorry to break it to you, but it does. At least, when it comes to ships it does. And man oh man, have the Borg got you covered in that department. Their design philosophy is essentially: "Fuck it, we'll just make a huge cube. We're in space, so who cares?" Indeed. So they basically run around in giant cubes 3km across. And it's not like they have five or six of them, oh no... they literally have shitloads of these floating around space, just waiting for some fresh assimilatable alien ass. Just one of these ships took down 40 top of the line Federation vessels without breaking a sweat. Imagine a hundred thousand... or more.
And it's not like they have some retarded design flaw that would let a tiny one-manned fighter screw everything up. Who the hell does that? No, everything on a Borg ship is decentralised. In other words, everything is everywhere. Their ships literally have no weak points. You could physically destroy almost 80% of a Borg Cube and it would keep going at you. Except that that won't happen, since they can regenerate their hull faster than you can pew pew at them. In a space combat against these guys, you're pretty much fucked ten ways 'till Tuesday.
That's basically the Borg's motto. "Assimilation!" They are after what they perceive as "perfection", and the most logical way they found to go after that was to merge with everything else in the known universe through technology. This was probably decided on a lazy Saturday afternoon a few thousand years ago, after having a bit too much to drink. Regardless, all they want now is for you and everyone/everything that has ever existed to be merged into the collective, and all it will cost you is your individuality! But it's not like you were using that anyway. Besides, once you become a drone, you'll get kick-ass prosthetics like an eye implant that can see in, well, blurry green, and a cool arm replacement that will be completely useless!
It opens cans and nothing else!
Also, lasers. There must always be lasers. Lasers are cool.
This is essentially what makes the Borg unstoppable. Because, let's face it, on foot the Borg are pretty slow, and they're really easy to shoot down. Like zombies. Well, they've got one more trick up their cybernetic sleeves. Adaptive shielding (something zombies are still working on). Basically, you shoot down a few Borg drones, and after a while you stop. Why? Because while you were gunning down about one trillionth of their total population, the Borg were analysing the specifics of what you were shooting them with, and now they've adapted. And that's that.
Unless you get "creative"
Ok, you tell me, if the Borg are so freaking unbeatable, how come they keep getting their asses kicked in the show? To which I can only reply: lazy writing. Don't get me wrong, at first the Borg were a real menace, and it was really hard to make it out alive from an encounter with them. But then Voyager came around, and ruined everything. The Borg were practically reduced to an alien-of-the-week deal, and were way too easily defeated. The writers, in their infinite wisdom, decided that if they slapped some tits on one of them, we wouldn't notice the drop in quality... and, well, they were right, for the most part.
What's a plot?
We can only hope that the Borg will one day return to their former terrifying glory... but we probably won't get to see that for a while, since the franchise has apparently decided that Kirk and Spock are "in" for the time being. I guess we'll have to wait for the Michael Bay Next Generation reboot in 2025...