Darth Vader is without a doubt one of the most beloved and feared villains in all of fiction, right up there with characters like The Joker, Hannibal Lecter, and Lord Voldemort. Nowadays in the realm of science fiction, it's practically impossible to find someone who doesn't take at least a little influence from the dark lord of the Sith.
Even his fellow Sith Lords are guilty of the cyborg ruler's influence, no matter where they come from. Whether they originate in the expanded universe or the current Star Wars canon, Vader has more than a few copycats out there. Imitation is the sincerest form of flattery, right?
Count Dooku
Count Dooku was easily one of the highlights of the often bashed Prequel Trilogy, but that's partially thanks to the incomparable Christopher Lee. That being said, there's at least a healthy helping of Vader's influence to his demeanor, appearance, and presence, along with a little of Lee's Dracula. His calm and calculating nature, his long black cape, his deep voice, and powerful presence all call back to the original face of the Dark Side.
Darth Tenebrous
A member of the Star Wars extended universe, Darth Tenebrous was a precursor Sith Lord along with Darth Plagueis who shared more than a cybernetic breathing apparatus with Lord Vader. Not only was the character quite a powerful member of the Sith, but his attitude towards the Force is remarkably similar to that of Vader's.
As a prolific scientist, Tenebrous saw the Force as a tool for progress instead of a power source alone. The ability to destroy a planet is insignificant next to the power of the Force, after all.
Darth Maul
With the introduction of the Star Wars prequels, George Lucas and his team had the daunting task of creating a new villainous threat. Since Anakin Skywalker was barely old enough to have a pod-racer license, the film needed a new threat. Enter the frightening and agile Darth Maul. Though he lacked the cybernetics, his character was absolutely drenched in shadow and mystery. And once fans saw him fighting the Jedi, it was just like seeing Vader unsheath his lightsaber for the first time.
The Grand Inquisitor
The Inquisitors are to Vader as the Death Eaters are to Voldemort. With their black leather attire, masks, and appetites for death and destruction, it's easy to see who they admire most. But none imitate the Sith Lord himself better than The Grand Inquisitor.
He's as commanding, callous, and cruel as Vader, and even without his helmet, he bears a familiar resemblance to him unmasked. The only place he exceeds him is in pure intimidation.
Darth Nihilus
Although his approach is far more ethereal and wraithlike than Vader's, Darth Nihilus is where the lines begin to blur. Like Vader, Nihilus was once a man, but due to being seduced and consumed by the Dark Side of the Force, he has become something else entirely. A manifestation of the Dark Side itself, Darth Nihilus hungers for Force energy, walking as an avatar of fear and suffering.
Galen Marek (Lord Starkiller Ending)
The Force Unleashed was a hack-and-slash video game where players got to assume the role of a Sith apprentice under Darth Vader himself, but they were also given the choice to chose the good or evil side of the force.
If they chose the path of the Dark Side, Galen Marek would essentially become Darth Vader's replacement under the Emporer. Galen even gets fitted with his own Vader-inspired suit and mask to top it all off.
General Grievous
Grievous is more than just a cyborg with a breathing problem, but a figure foreshadowing the future of a certain Jedi. He was a Kaleesh who was so consumed by hatred and pain, he gave himself over to the Dark Side to be Dooku and Sidious's sinister cybernetic project. More machine than organic being, Grievous is the Sith's perfect killing machine, slaughtering supposedly 100 Jedi by his hands. All of this is eerily similar to the fate that would befall Anakin Skywalker.
Second Sister
The Inquisitors have already been mentioned once, but Second Sister's appearance, manner, are so blatantly ripped off of Darth Vader, it's almost like she's practically the embodiment of genderswapped fan art.
Joking aside, Second Sister is nothing to sneeze at. Although she lack's Vader's control and distinct presence, she is a deadly villain to be tangoed with in Jedi: Fallen Order.
Darth Revan
If one looks at the Star Wars series from a timeline perspective, Darth Revan was Vader before it was cool. This Dark Lord of the Sith essentially laid out the basics for Vader in a few easy steps. Betray the Jedi, don a cool and intimidating mask that strikes fear into the hearts of your enemies, create a sizable body-count with the Dark Side of the Force, and leave a legacy of fear behind for future Darths to follow. It's as simple as that.
Kylo Ren
There is no bigger fanboy of Darth Vader in this or any other galaxy than his grandson, Ben Solo. Taking up the moniker of Kylo Ren, an angry young apprentice attempted to follow in his grandfather's footsteps by walking the way of the Dark Side. From the familiar-looking mask to the edgy black attire, Kylo is desperately trying to be Vader reincarnated. If only he could control that temper of his.