In the fourth season, Seven of Nine (Jeri Ryan) reveals that the Borg never assimilate the Kazon, whom they refer to as species 329 and "unworthy of assimilation" due to a belief they would "detract from perfection". A Kazon crew member was included on a holographic reconstruction of Voyager as a warship.
The Borg quickly realize that Species 8472 is immune to assimilation and that contemporary Borg technology is no match for it. In fact, 8472 is one of the few species so advanced that its ships are able to destroy Borg cubes in seconds.
Species 329 name themselves as Kazon, a relatively primitive, yet warp-capable species with a limited understanding of the technology in their possession which they gained control of after conquering their former oppressors.
According to Seven of Nine, there's a simple reason for this; the Borg encountered a Kazon colony, but "their biological and technological distinctiveness was unremarkable." It seems the Borg considered the Kazon to be "unworthy of assimilation." In their view, the Kazon would not add anything to the Borg Collective; ...
Zero (Angus Imrie) merged his mind with the Borg Collective, but his attempt to ask for a solution about the Living Construct fell on dead ears. Instead, the Borg quickly assimilated Zero.
The Hirogen see every approach of prey to defend themselves as incentive. The Borg see every species worth assimilating as prey. The Borg can adapt, Hirogen never give up. We see multiple Hirogen as assimilated Borg.
Tuvok, B'Elanna Torres, and Captain Kathryn Janeway have been assimilated as Borg drones aboard Tactical Cube 138, but their personalities remain intact thanks to a short-lived anti-assimilation inoculation developed by The Doctor.
If they thought the Xenomorphs biological uniqueness (rapid growth, silicon based biology, acid blood etc) was worth assimilating, they would likely succeed. Being non-technological the Xenomorphs would be helpless against a determined Borg attack.
According to Hugh (Jonathan Del Arco), the former Borg from Star Trek: The Next Generation who is now Director of the Romulan Reclamation Project, the crew of the Shaynor were the only Romulans ever assimilated by the Borg.
Officially, the existence of a Borg homeworld is a hypothetical one as it has yet to be revealed in a canon Star Trek source. However, an identity for the Borg homeworld is given by the non-canon novel Lost Souls, which names it Arehaz.
The society of species 1 consisted of female drones and a queen for each colony. There were males, but they were only used for mating, much like ants on earth. Through millions of years, species 1 developed a collective consciousness on the colony level, with each colony having its own.
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.
The Borg would become an unstoppable force, able to assimilate or simply change the galaxy to fulfill their vision of perfection. In addition to gaining all the knowledge of a particular Q, having their consciousness uploaded to the Borg collective, they would also potentially gain the god-like power of a Q.
Reputation aside, there is little in the Ferengi for the Borg to wish to assimilate. Physiologically, they are fragile creatures and do not have traits that the Borg are interested in. Aside from a passing interest in their auditory capabilities, it's doubtful the Borg would assimilate the Ferengi.
The A.I. at work in Star Trek: Discovery has employed similar methods, has a similar motive, and even shares style points with the Borg. It's a technology-based entity that despises all organic life and seeks to have the galaxy all to itself (and it's also currently thwarted by humans).
But we never saw non-humanoid Borg in the shows and movies. We know that they tried to assimilate species 8472, but it failed because of their biology being resistant to the nanites.
Yes, there is good reason to think that the Borg can assimilate some androids and robots. The Borg are able to “assimilate” much of the Enterprise's systems in First Contact.
Could "Q" or a "Founder" be assimilated by the Borg? Yeah, maybe. There seem to be some species like species 8472 though, that are just incompatible with assimilation, but for the most part they can usually find a way to assimilate many species.
The pathogen completely decimated the Borg Queen and left the entire Collective in ruins, and while Picard and the crew of the USS Enterprise-D dealt the final killing blow, it was Janeway who originally crippled the Collective permanently.
He was mentioned once by Deana Troi (who did appear, but never met Janeway) in the episode “Pathfinder”. The only time Picard and Janeway talk onscreen is in Star Trek: Nemesis when Janeway, having been promoted to vice admiral, gives him his assignment.
While fighting off the Borg near main engineering, Data was captured and brought before the Borg Queen. Instead of attempting to assimilate Data, she embarked on a transformative process to make Data more "Human," by attaching Human skin onto his android skeleton.
