Star Trek has featured a plethora of intriguing villain species over its seven-decade run, as well as a few that haven't quite worked. As humans sought out new life and new civilizations in Star Trek's future, they were bound to run into some that weren't exactly friendly. The antagonistic species the crew of the USS Enterprise and other Starfleet vessels encountered were often reflections of humanity's own past failings, with fascist regimes and violent dictatorships in full swing across the galaxy.
Not every Star Trek story features a cut-and-dry alien villain; sometimes Starfleet officers face aliens with whom they simply have a misunderstanding in good faith, like Captain Jean-Luc Picard (Patrick Stewart) and an unintelligible Tamarian captain in the classic Star Trek: The Next Generation episode "Darmok," or the misunderstood creature called the Horta encountered by Captain James T. Kirk (William Shatner) in the Star Trek: The Original Series episode "The Devil In The Dark." That said, most of Star Trek's iconic antagonists are alien species with clear hostility toward the idealism Starfleet and the Federation stand for. Here's a ranking of every major Star Trek villain species.
10 The Kazon
The Kazon were supposed to be Star Trek: Voyager's ultimate bad guys, but it didn't quite work out that way. A warrior race bent on conquest, they generally came across as cheap knockoffs of the Klingons. Even the Borg didn't want to assimilate them. The Kazon would unsuccessfully menace Captain Kathryn Janeway (Kate Mulgrew) and the crew of the USS Voyager for the show's first two seasons before being phased out of the show in the season 3 premiere "Basics, Part II." Star Trek: Prodigy has somewhat reimagined the Kazon as slave traders, but they remain one of Star Trek's most underwhelming antagonists.
9 The Ferengi
The Ferengi were conceived by Star Trek creator Gene Roddenberry to be TNG's primary antagonists, but their initial appearance in the season 1 episode "The Last Outpost" scuttled that plan. TNG would eventually find their true villains in Q (John de Lancie) and the Borg, with the Ferengi only appearing sparingly over the rest of the series, often as comic relief. The Ferengi were eventually salvaged when Star Trek: Deep Space Nine refocused them as ultra-capitalists, largely through the duplicitous station bartender Quark (Armin Shimerman). Still, even if you owe them Latinum, most Star Trek species aren't particularly intimidated by the Ferengi.
8 The Breen
The Breen began as something of a running joke on TNG, often referenced but never actually seen. No one was laughing when they eventually made their debut on DS9. A brutally violent species that wear fully enclosed atmosphere suits to acclimate to warmer climates, the Breen Confederacy became a major galactic player in the final days of the Dominion War, aligning themselves with the Changelings to take on the Federation, even managing to destroy Captain Benjamin Sisko's (Avery Brooks) ship the USS Defiant. The fearsome Breen returned in the Star Trek: Lower Decks season 3 episode "Trusted Sources," having overrun the planet Brekka. The Breen remain a mysterious, potent threat to Starfleet.
7 The Gorn
The Gorn are a lizard-like species that debuted in the TOS episode "Arena," where Captain Kirk was forced into a fight to the death with one of the monstrous creatures. One of the few purely malevolent species in Star Trek, the Gorn have been referenced often over the years but only made their return recently in Star Trek: Strange New Worlds. The family of the Enterprise's security chief Lieutenant La'an Noonien-Singh (Christina Chong) was murdered by Gorn when she was a child, and she was forced to relive her trauma when the Enterprise crew had to face a group of homicidal Gorn hatchlings.
6 The Jem'Hadar
The Jem'Hadar were a genetically-engineered warrior species that served as the Dominion's primary military force during the Dominion War. Physically powerful and with little regard for their own lives, they were bred to be the ultimate killing machines. They were addicted to the drug ketracel-white, and their dependence on the drug provided by the Dominion kept them in line. While unintended by their Dominion masters, the Jem'Hadar developed a somewhat twisted sense of honor; they weren't true believers in the Dominion cause, they simply had no other choice but to obey. In many ways, the Jem'Hadar were as much victims of the Dominion War as anyone else.
5 The Romulans
One of Star Trek's oldest enemy aliens, the Romulans debuted all the way back in the TOS season 1 classic episode "Balance Of Terror." Since that initial stunner of an episode, the insidious Romulan Star Empire remained a constant threat to the Federation. They maintained something of a cold war with the Federation during the TNG era until their home planet, Romulus, was destroyed by a supernova in the late 24th century. After the destruction of Romulus, the Romulan Empire was severely weakened, but they were still powerful enough to send a 200-ship-strong armada to attack a planet of synthetics in the Star Trek: Picard season 1 finale.
4 The Changelings
The Changelings, also known as the Founders, were the primary species behind the Dominion. The rulers of the Gamma Quadrant naturally reverted to a gelatinous goo form, and generally congregated together in a massive sea known as the Great Link. Condescending and arrogant, the Changelings believed humanoids were inferior life forms and referred to them derisively as "solids." They sought to annihilate the Federation in the Dominion War through both direct attacks and complicated subterfuge. They were eventually defeated when Starfleet infected the Great Link with a deadly virus, which was ultimately cured when the benevolent Changeling Odo (Rene Auberjonois) rejoined the Link at the conclusion of DS9.
3 The Klingons
Since TOS season 1, the Klingons have been portrayed as a warrior race obsessed with honor and conquest. The Klingons have looked quite different over Star Trek's history, though the most frequent appearance has been the one made famous in TNG, featuring pronounced head ridges and leather and chain battle gear. By the era of TNG, the Klingons were somewhat uneasy allies of the Federation, though that alliance was temporarily dissolved in the lead-up to the Dominion War. Whether friend or foe, the Klingons are always a volatile, unpredictable species who yearn for battle and glory, often against the wishes of the Federation.
2 The Cardassians
The Cardassians were introduced in TNG, but they became an iconic villain species on Deep Space Nine. Long an enemy of the Federation, the Cardassians had declared an uneasy truce with the Federation by the time of DS9. During their years-long conflict with the Federation, the Cardassians occupied the peaceful, non-Federation planet Bajor. The Cardassians brutalized the Bajorans, perceiving them as an inferior race suitable only for slave labor. Charismatic monsters like Gul Dukat (Marc Alaimo) saw these atrocities as the necessary cost of building an empire, indifferent to the plight of the innocent. The ultimate treatment of the Cardassians at the hands of the Dominion was a bitter irony.
1 The Borg
A species of faceless, voiceless cybernetic zombies, the Borg are the most threatening villains in all of Star Trek. Introduced in the TNG season 2 episode "Q Who," the Borg assimilate entire planets and species into their collective with the explicit goal of making all of the universe Borg. After assimilating Captain Picard in the TNG season 3 episode "The Best Of Both Worlds," the Borg decimated Starfleet at the battle of Wolf 359 on their way to assimilate Earth. The Borg were critically wounded in the Star Trek: Voyager series finale "Endgame," but they remain the most dire threat to humanity in all of Star Trek.