Over twelve years, and seven seasons, the value it adds to the films and broader canon, are all notable aspects of the animated show, and so too are the many characters it dives into.
Both original characters and those from the movies appear in the show, and just about all of them get improved due to their inclusion. As is always the case with Star Wars characters, some are lovable people, some are abhorrent, and some fall somewhere in between.
Count Dooku
This entry pretty much counts for both Dooku and Palpatine; however, the latter is far more prominent in the show and is doing the bidding of Palpatine anyway.
Count Dooku is one of the main characters made infinitely better by this show as audiences get a good look at his skill, power, and how he manipulates and rules the Separatists. He is infinitely suave, and outmaneuvers the heroes constantly, and is, all in all, pretty evil, if not wholly misled.
Maul & Savage
If you told someone who just saw The Phantom Menace thirty years ago that Maul not only survived Obi-Wan cutting him in half, but found his brother, created a crime organization, and lived through the events of the Clone War, they would call you crazy.
Alas, that is what happened, as the magically powered brute Savage Opress found his brother in a state of manic, fixed him with the help of Mother Talzin, and the two went on a rampage. Maul outlived his brother, and how he is fueled by raw hatred and anger is a different type of evil from Dooku, but one that still makes him loathsome from an in-universe perspective.
Asajj Ventress (& The Nightsisters)
Count Dooku's apprentice was a character retained by the 2008 version of the show from the 2003 incarnation, and Asajj Ventress continually proved herself to be a great character and villain.
Going from assassin to Nightsister to bounty hunter, Asajj has quite the journey in the show. She gets aided a lot by fellow Nightsisters and their leader Mother Talzin, a character who is also extremely unlikable. What Asajj has going for her over other villains is sympathy, and had Dark Disciple got taken into consideration here, she would have likely ranked much higher.
Mace Windu
Mace Windu is a great character, who fills the role of uber-traditional, uptight, rigid, and emotionless Jedi who stringently follows the Jedi Code well. Still, it is also a character that is hard to like.
His treatment of Anakin was bad enough, already making him the most unlikable Jedi on the Council, with, to some fans, close competition from Luminara and Ki-Adi Mundi. Then there is his unbelievable hubris, which is, at times, infuriating, especially when you consider the possibilities if he had not treated Ahsoka like she was nothing to him in their last hologram meeting.
Anakin Skywalker
One of the best things The Clone Wars managed to accomplish was to improve on Anakin Skywalker in every conceivable way, from his character to his romance with Pé to his eventual downfall; it is all enhanced by this show.
He is also far more likable in this show and becomes so easy to understand and sympathize with. There is an argument that Anakin deserves a place further down on this list, but between his insane jealousy, and deep-rooted emotional issues, and violent tendencies that creep up, which he makes minimal effort to fix, he lands here on the likability scale.
Obi-Wan Kenobi
Obi-Wan Kenobi is a highlight of everything he is involved in, from the original to prequel trilogy, and across both animated shows. However, he may be at his most unlikeable in The Clone Wars.
You cannot dislike Obi-Wan, he is excellent, and he is likable. However, the mistakes he makes in the show, such as lie to Anakin, not believe in Ahsoka, and become another Jedi so insanely loyal to the Jedi Code and the war, do make him a tad less enjoyable as a person, even with his good heart and intentions.
Yoda
It was difficult to imagine how The Clone Wars could add to Yoda's character. He was 900 years old; surely, fans had seen his character in its entirety at that point in his life through the prequels and original trilogy.
What the show did, though, was truly build on the kind nature of Yoda. Fans knew he was a good person, but how he cares for Ahsoka, for Padawan's, for clones, life, and the Force, it all comes to a head in this show. Even if he makes some mistakes in of the war, Yoda remained one of the most likable Jedi.
Plo Koon
In the vacuum of The Clone Wars, though, there is only one Jedi who can come out on top as the series' most likable, and that is everybody's favorite Kel Dor, Plo Koon.
Plo Koon pretty much solidified his legacy as a fan favorite with the line "not to me." On top of that, though, Koon's general kindness and patience, as well as his relationship with Ahsoka and him being the only Jedi Master who believed even slightly in Ahsoka, makes him one of the show's best. Other good-hearted, thoroughly likable Jedi in the show include Kit Fisto and Aayla Secura.
Pé Amidala
Many people took issue with Pé in the prequel trilogy, citing performances and writing as the main reasons why the character did not work, despite Pé herself being pretty great in concept.
The Clone Wars rectified many of these issues. Despite quite a few arcs involving Pé being subpar, the character herself was memorable. Strong-willed, ionate, fearless, and intelligent, Pé was just like her daughter, and she was both likable and inspiring.
Satine Kryze
Any time the Mandalorians and Mandalore got involved in The Clone Wars, it made for excellent and exciting television. At the forefront of that was the Duchess of Mandalore, Satine Kryze.
She added so much to the show during her few times on it. She not only helped Mandalore become so brilliant from a fan enjoyment standpoint, but she added to Obi-Wan's character too. Satine was a pacifist who wanted to do good by her people and managed to stay kind yet strong.