I worded a recent question poorly, and seek to ask a similar but different one. Q isn't omniscient, but he is omnipotent. So, while he may not know everything about the Milky Way or the universe in general, we can assume he knows a great deal. In the galaxy, we have vast powers. The Dominion, Borg, Hirogen, Species 8472, et al. are powerful and imposing threats to the Federation. None of them had encountered any Federation ships prior to the Star Trek: The Next Generation episode Q-Who. And yet out of all these and more, Q chose to expose the Enterprise to the Borg. What qualities of the Borg resulted in Q picking them? What lessons does the Enterprise learn by interacting with the Borg that they wouldn't learn from some of these other foes?
The Borg are the very opposite of the Federation. The Federation extends through diplomacy and research, values every single life individually and tries to perpetuate every single culture and cultural aspect. The Borg do the exact opposite of this, they assimilate, destroy, and assimilate some more.
By introducing the Federation to the Borg, Q showed them their counter-part in the universe. A very dangerous counter-part which could destroy them. As I remember the episode correctly, he also told them a little bit about the Borg so that Picard got curious about them. This also brings Federation into a very dodgy situation, on the one side they are dedicated to peace, diplomacy and research...on the other side the Borg just want to assimilate or destroy them. For an omnipotent being that might be a curious thing to watch.
As Keen pointed out himself, there's also the possibility that Q wanted to show them not only their counterpart, but a mirror. From the outside, the Federation can be perceived as a very daunting organisation, not much different to the Borg. If you're a small and independent world and culture, joining the Federation can be a daunting thought, because you might lose yourself in that big community completely. Your culture, way of life and thoughts may be drowned by the big Federation and you become a part of it. This is not much different to the Borg.
Also, from a more story-telling aspect are the Borg a perfect fit into the Star Trek Universe. Federation, Klingons, Romulans and Ferengi all highlight simple aspects of human nature, diplomat, warrior, spy, trader. The Borg are a very harsh contrast to that universe and bring in the aspects of a machine.
Originally, the aliens seen in Conspiracy were going to be the first wave of Borg. This idea was abandoned later on, when the Borg were switched from insectoid to cyborg. So the Borg were indeed going to appear before Q's interference. Not to mention the whole scooping-up-cities thing @Kyle mentioned in the comments, which was referenced in Q Who when in Borg space.
As for the others you've mentioned?
- The Hirogen are about as far from a cohesive group as you can get, and not very advanced compared to the others. It's unlikely they could get to the Federation in large enough numbers to be a threat.
- Species 8472 didn't know of the existence of our dimension until the Borg interfered.
- The Dominion already knew about the Federation, and were making plans to deal with them around 2570, when it was predicted the Federation would reach Dominion space. The discovery of the wormhole just accelerated events.
The Borg were already on their way, and no one knew. Largely due to the (retconned in) events of Regeneration.
I'll offer a speculative but plausible answer: de Lancie Q wasn't attempting to punish the Enterprise, its crew, or the Federation. He chose the Borg not because he wanted to screw with them, but rather give them a headsup a few years in advance so that they might have a fighting chance against them.
For his own reasons, he can't just play this as a nice guy. Perhaps the rules of the Continuum permit him to torment but not to help, or the logic for their rules may even be incomprehensible. But being a deity-or-near-deity-level entity, he also cheats these rules occasionally. And it seems he does favor Picard and perhaps all of the Enterprise crew.
In another episode where Picard has a near-death-experience, de Lancie Q reveals himself as God, and rather than screwing with him he makes an effort to give Picard insight into himself. It is a hard lesson, but not one needlessly abusive. Picard himself speculates with Riker that he doesn't know if it was real or a hallucination, but if it were the former then it seemed out-of-character for Q.
Now, I have big issues with the writers for this show. There were great episodes and then "not great" episodes, as we're all aware. So I hesitate to credit them with something so subtle and pleasant as this, but if we interpret the various Q episodes in the best light possible, then I assert that we must consider that this was what they were attempting to do all along: to make Q a mysterious, nuanced, and ultimately covert force for good.
If all of this were true, then Q would find himself in a predicament. He can't just wave a wand and fix it for the Federation (he's not permitted), but his intellect sees what is plainly obvious (at least to him): the Federation will be curbstomped into nothingness in very short order. None of his other tricks will suffice. The things he might do to other races and powers to put obstacles in their path or divert them, these will will not stop or even much delay the Borg. The Federation has to win it (mostly) on their own. The only possible help then becomes to make them aware of it early in the safest manner possible. Does he drop them into the middle of a Borg nest with hundreds of cubes? No. He puts them in range of just one, in such a situation that the Enterprise can glean some useful intelligence and escape to fight another day. And if I'm not wrong, finally he hopes for the best.
From what I remember of the episode, Q and Picard were discussing humanity’s exploration of space, and Picard was arguing that humanity was capable of learning to handle whatever it found, without help or oversight from Q. Q took umbrage at Picard’s arrogance, and wanted to show him something that humanity was utterly unequipped to deal with.
The Borg are perfect for this. They’re pretty different to the threats previously encountered by the Federation. As I think Guinan said in the episode, they can’t be reasoned with. They adapt to every method you use to fight against them within minutes, and they don’t want anything except to absorb you and your entire race into their zombie-like collective.
When the single Borg ship that the Federation encountered that day reached Federation space, it destroyed 39 ships in a single battle, and was on the verge of taking over earth. I don’t think any other species that we’ve seen since in Star Trek could have done the same.
But equally, this encounter did not lead to the Federation’s destruction, or to a long, drawn-out war. The Borg don’t particularly care that the Federation didn’t fall to them on that occasion; they just continue assimilating as before, they’ll get to the Federation eventually (time-travel shenanigans notwithstanding). I think this was important to Q, because he wanted to scare Picard (and, by proxy, humanity) about the horrors that their galaxy contained, in order that they be better prepared for exploration.
And when you consider, for example, how useful the Defiant (developed in direct response to the Borg) was in the Dominion War, you could argue that Q’s plan was a roaring success.
The Borg are so very different from the Federation and from anything the Federation has experienced that they represent something frightening at its core. Being subjugated by an external alien power is a familiar thing to the Federation even if personal experience with that is mostly lacking. And under such conditions it is always possible to fight back, to rebel, to overthrow, to subvert, or to even live free within one's own mind. But this is not true with the borg. The borg assimilate, they wipe out cultures and individuals utterly and bend them to their own purposes. This is not something that can be rebelled against and as such it is a very much different sort of threat than anything the Federation is used to.
And that was something Q wanted to show the crew of the Enterprise, that even the galaxy (let alone the Universe) holds threats that the Federation can hardly conceptualize, let alone cope with.
Additionally, the borg already had contact with the Federation (even within the isolated canon of TNG at the time). They were responsible for the missing Romulan and Federation colonies and outposts in the episode "The Neutral Zone" which takes place a full year prior to "Q Who". Additionally, Guinan's people had already had contact with the borg (as established in that episode). It was only a matter of time before the borg and the Federation would come into conflict, Q merely sped up the timetable a little. In a way Q was doing the Federation a favor, since without such knowledge of the borg threat they would have been even less prepared.
I think it's more of the Q being afraid of what the Borg could accomplish. In the Voyager episode where his son brings 3 cubes to Voyager (to mess with Janeway), de Lancie Q shows up and says
How many times have we said this? Don't provoke the Borg!
That indicates that possibly, the Borg were known to be a potential threat to the Q in the long-term of the universe's timescale. The Borg may or may not have been aware of the Q and their power - and if they did find out about them, they would be a huge target. The Borg would like nothing more than to have the "perfect" powers and knowledge of the Q.
Also, Q likely knew that the Federation would play a major role in either curbing or halting the advance of the Borg, or even in removing the threat altogether one way or another. That outcome would only be achieved if they had time to prepare - which is what happened in
If that hadn't happened, Wolf 359 would not have happened. That means Sisko would not have been grieving for his wife whom he lost there, and possibly would not have been in a position to become commander of DS9 when that happened. With that, a couple things could have happened:
- They never even find the wormhole, and thus exploration of the Gamma quadrant never happens and the war with the Dominion never happens
- They DO find the wormhole, and the Dominion ends up invading and winning their war because 'The Sisko' is not there to ask the Prophets to do something about the invasion force.
In the first instance, this means the Federation, Klingons, and Romulans don't develop the technology, weapons, and tactics that contributed to the defeat of the Dominion/Cardassian alliance. So when the Borg finally do attack, the Federation would be significantly behind their current in-universe counterparts, and thus less able to defend themselves. Plus they would have less knowledge of the Borg itself, including its vulnerabilities and strengths.
In the second instance, it means the Borg find the Dominion in the Alpha quadrant instead of the Federation, and Cardassians, and in the Beta quadrant, Romulans and Klingons. The Borg would win against the Dominion, especially once they assimilate them. They would essentially be able to assimilate the entire galaxy at that point.
Some other thoughts on why the Borg. They're near enough to the Federation be a threat, without the wormhole the Dominion is too far away, and Species 8472 haven't become aware of our reality yet, the Borg would follow them home. They are an existential threat to the Federation, the Hirogen can be a threat to one ship, but a fleet with full support, not so much. And they would blatantly rub Picard's nose in just how outclassed humanity was, the aliens from Conspiracy are too subtle for that.
Simplest explanation is the Star Trek writers had not yet created/thought up The Dominion, Hirogen or Species 8472.