Of all the wonderful villains in the Star Trek franchise, there is possibly none more memorable than the mysterious Borg. They assumed the role of the main antagonists for The Next Generation, but appeared throughout almost all the following shows in similar nefarious ways.
The Borg are unlike other franchise villains like the Cardassians or the Original Series Klingons. The biggest fear surrounding them was not that they would come and kill, but that they would kidnap and mutilate until whomever their victims are turned into mindless drones. They do not conquer; rather, they absorb others into their collective. This is the biggest character trait for the Borg, as their catchphrase is literally ‘Resistance is futile, prepare to be assimilated.’ However, there are some lucky few that are immune to their assimilation.
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The Q are potentially the least likely to get assimilated, for not just one reason but two. Firstly, their near omnipotence and god-like powers would make it almost impossible for a Borg, or even all Borg working together, to capture a Q. Members of the Q continuum, like John de Lancie’s iconic Q, have shown time and again that they are capable of ridiculous amounts of power. They can change even the fabric of reality, tearing timelines apart, all with just the snap of their fingers. The Borg would never stand a chance.
If a Q were, for whatever reason, somehow cornered and unable to use their powers to protect themselves, there is little chance that Borg assimilation procedures would work. The Q may look organic when they present themselves to various Starfleet captains, but it’s important to remember that these are just projections of their true selves in a form that ‘mortal’ eyes can comprehend. They are non-corporeal beings of energy. In reality, there would be nothing for the Borg to actually assimilate. It would be like trying to put implants into a hologram — it would not work.
This bunch of nasty, xenophobic alien monsters are potentially the most famous to be immune to Borg assimilation attempts. They play a fairly large part in the plot of the Voyager series. Their name is unknown. 8472 is a designation that the Borg gave to them when they discovered and tried to assimilate them in another dimension known as fluidic space.
The Borg are always trying to find and take new technology and biological distinctiveness to achieve their version of perfection. Species 8472 were not only powerful apex predators, but they also possessed advanced biotechnology. Their uniqueness made them a prime target, but much to the Borg's dismay, they turned out to be immune to Borg nanoprobe technology. Unable to assimilate or defeat them, the Borg fled, resulting in their entire race nearly getting wiped out until Janeway stepped in. It might be possible that Janeway's manipulated nanoprobes (the ones she technically uses to commit acts of criminal behavior) could have been used to assimilate 8472. However, the crippled Borg never attempted it again, most likely believing it wasn't worth it.
The Tholians are a crystaline based life form, and one of the few non-humanoid aliens in the entire franchise. They make appearances in The Original Series and Enterprise, and are great examples of aliens that live in a completely different way to any other life form encountered. They truly feel alien in a universe where many aliens feel normal. Due to them being crystaline rather than biological, the Borg would have trouble assimilating them. The Tholians no iron, copper, or cobalt-based blood like other easily assimilated races, so nanoprobe technology would not work.
The Borg, however, are ever evolving and painfully smart (ignoring their creepy Achilles heel), so they might be able to work something out. It’s not canon, but it's not ruled out either. The issue then becomes that of keeping the assimilated Tholian alive. Tholians need a temperature of a whopping 400 degrees Kelvin to stay alive. Without it, their silicon-based "skin" cracks and splits, resulting in a painful death. This creates a unique problem for the Borg. With a newly assimilated Tholian, they would need to home them. A Borg cube is far too cold for a Tholian to survive. The Borg could theoretically create special sections or entire cubes that run hot, but the effort is most likely not worth it, especially when the species offers little to the collective.
These three species in the franchise are, theoretically, immune to Borg assimilation. There are many others, however that are immune in a way: they are simply not worth the Borg's time. The Borg have their own twisted form of the prime directive. They only add species to their ranks that they think will help improve them, helping them reach perfection. Some races however, like the Kazon and the Pakled (especially the Pakled), would detract from this. The Borg believing that assimilating them would muddy the water. It seems the best way to avoid becoming a Borg drone, if one lacks crystaline biology or a natural immunity to nanoprobes, is to lack intelligence or usefulness. The Borg leave such beings alone.
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