The Star Trek universe is filled with individuals who are, simply put, up to no good. Sometimes it's individuals who cause problems for the heroes of each series. Other times, it's entire races who just can't help themselves. Their status even changes from series to series.
Back in the era of The Original Series and Enterprise, the Klingons were straight up villains. By the time of The Next Generation, the Klingons had become allies of the Federation. For the most part, at least. Nefarious Klingons like Lursa and B'Etor, as well as the House of Duras in general, were still causing problems.
Other species like the Romulans seem to consistently be a thorn in the Federation's collective sides. Even when they team up with Starfleet, the Romulans are still plotting against their temporary allies. That might seem paranoid to some, but it's only paranoia when someone isn't out to get you. In the case of the Romulans, they should probably be more concerned about the Klingons.
But when you step back from well known villains like Klingons, Romulans or the Q, there are others who maybe haven't gotten the respect they deserve. Some of those villains have had more impact than others but were still incredibly memorable.
Ian Goodwillie is a freelance writer based out of Saskatoon, SK, Canada. He has a BA in English Literature from the University of Saskatchewan and completed the Writing Program at Vancouver Film School. More importantly, Ian is also a wrestling fan, comic book reader, video game player and photographer. He loves to write and writes about what he loves. Ian's also an unrepentant, unapologetic Cougar Town fan, a show he will defend until the day he dies.