Between Obi-Wan Kenobi, Lords of the Sith and various upcoming projects, the iconic villains and Sith Lords of Star Wars, such as Darth Vader, are getting some serious time to further their legacies as incredible characters. There are many Sith and dark side Force users, though, who do not get even half the love a Vader or an Emperor gets.
That is not to say those two Sith Lords can be touched in terms of dark side Force using villains, but between the canon and Legends timelines, there are a variety of characters who deserve more respect, undervalued by fans, creators or both.
Maul
Maul's character is interesting in regards to deserving more respect or being underrated, considering that a portion of the canon junkie fanbase would argue he is overused. Yet, for those unaware of his journey through the extended canon, Maul deserves more respect.
Everything Maul appears in, everything that furthers his character, has a brilliance to it, from the Duel of the Fates all the way to his appearances in Rebels. His backstory in the comics, the Son of Dathomir comic run, and his return in The Clone Wars, are all aspects of Maul's character that showcase how great he is, highlighting both his intelligence and his animalistic ferocity. The impact Maul has had on the stories of Obi-Wan, Ahsoka, Kanan, etc., also deserves to be recognized more; he is a crucial piece of the canon puzzle.
Trilla Suduri
It can be said that the Inquisitorius as a group are underappreciated, perhaps most so the Grand Inquisitor himself, who is an articulate, powerful villain with a fascinating story. He is more well-known and appreciated than Trilla Suduri, though.
Appearing in Jedi: Fallen Order Content along with a few minor comic appearances, Trilla has not been used much in the canon yet has impacted those fans of the game. Witty and sinister, intelligent and talented, all while being the perfect antagonist to Cal and a genuinely threatening presence with emotional layers to dig through, Trillia may be the game's best character and one of the best villains of the Disney era, a Star Wars video game character who should come to live action.
Darth Plagueis
Pretty much every fan knows who Darth Plagueis the Wise is, but with his limited use in canon, many fans have not really gotten to appreciate how great of a Sith Lord he was.
Speaking on the Legends version of the character, which James Luceno wonderfully explores, Plagueis was instrumental in the events of the Skywalker saga due to his relationship with Sidious. A wildly intelligent individual working from the shadows, Plagueis knew how to manipulate the Galaxy to his will and had the power to do it by force if he was so inclined. His obsession with the manipulation of midi-chlorians was essential, and he clearly deserves more of a presence in official Star Wars canon.
Darth Marr
The Old Republic in Legends is an era filled with awesome, destructive Sith Lords, many of which get heaps of praise. One who does not get anywhere near the love of others is Darth Marr, a member of the Sith Empire's Dark Council.
Prominent in the MMORPG The Old Republic, Darth Marr is not just a strong, intelligent leader but fiercely loyal to the Sith Empire, someone who fought for the good of the Empire. He to bring together those divided, someone above (or who tried to be above) the senseless bickering and ego of the Sith, all while having an insanely cool and badass look. His story is one worth exploring.
Marka Ragnos
Marka Ragnos is one of those Sith Lords of old appreciated by many Legends lovers, but one with so much untapped potential that could be so great for stories in the ancient canon, who also may not be known to non-Legends admirers.
A ruthless and wildly revered Sith Lord, Ragnos oversaw the Golden Age of the Sith, an impressive feat of stability and relative calm of the Sith Empire and an important time in Sith history, with Ragnos ruling for a century. While his becoming a Force ghost does not work, his badassery must still be appreciated. Ragnos has pretty much everything one could want from a Sith Lord without quite as much acclaim as the likes of a Revan.
Tenebrous
While everyone is aware of the still underappreciated Darth Plagueis, far from everyone is aware of his own Sith Master, Darth Tenebrous, a Bith who is canon but has yet to really be utilized, just like his apprentice.
Outside of Figrin D'an and the Modal Nodes, Bith are an underused species in Star Wars, so it would be good to see Tenebrous used in the canon more often. But outside of that, he has some interesting and unique layers, such as his talent for starship design, thought to have designed Maul's Scimitar. A calculating, cold, logical individual with a conservative view of the Force and obsessed with molding the future, Tenebrous has a lot of potential and a lot of great character for any Sith fan to explore.
Ajunta Pall
In the canon, there is an unnamed figure known as the first Dark Lord of the Sith who broke away from the Jedi Order to tap into the unmined potential of the dark side. In Legends, that figure is Ajunta Pall.
Pall's separation from the Jedi Order led to the Hundred Year Darkness, and he is undoubtedly one of the most important figures in Galactic History, but also one who is immensely unexplored. It will be fascinating to see if the Star Wars canon ever builds on the mysterious rogue Jedi who was Ajunta Pall in Legends.
Darth Zannah
Darth Bane is a name well known within the Star Wars fandom, famous for his power and genius, as well as for creating the Rule of Two, he is one of Star Wars' best Sith Lords. His apprentice, Darth Zannah, though, also deserves some considerable love, more than she has gotten.
Zannah also exists in canon, and like Bane, her lack of any sort of story shows immediately that she deserves more respect. In Legends, though, she was still undervalued. Her power was immense, defeating Bane himself 1v1 and having gifts of Sith Sorcery. Perhaps most importantly, Zannah continued the legacy of the Rule of Two and the Sith legacy, being the first after Bane to employ the rule. She is a badass with such an interesting backstory, and her role in the incredible Darth Bane trilogy deserves more recognition.
Asajj Ventress
For many hardcore fans of The Clone Wars and Dark Disciple, Asajj Ventress will rightfully be seen as one of the best dark siders in the canon. To those outside of that group, though, the sympathetic Star Wars villain Ventress deserves more reverence.
Not only does Ventress have a fascinating and complex character journey that takes her from the light to the dark to the in-between, but she has immense skill so often overlooked. She held her own and survived battles against both Obi-Wan and Anakin, as well as Dooku, no easy feat.
Count Dooku
There is so much to adore about the character of Count Dooku, from the legendary Christopher Lee to the elegance of his lightsaber combat to his classy royal outfits. Yet, Dooku never seems to get the love he earns.
Dooku is one of the best lightsaber duelists in the Galaxy, a master of blade-to-blade combat, and is an incredibly articulate, intellectual individual. On top of all that, he is both student to Yoda and master to Qui-Gon Jinn, two fantastic Jedi with whom to have stories. Tales of the Jed will half-focus on his fall to the dark side, and fans of his cannot wait to see that story unfold.