1. The Master of Masters. I just love that archetype of the greater-scope villain who basically shaped the entire franchise for their own nefarious purpose that nobody is capable of understanding. Every time he's on screen, he has spectacular aura and charisma and manages to be funny, goofy, deep and terrifying at the same time. I love how easily he manipulates everyone with mere words and praises he knows they will take the wrong way and screw themselves over. Besides, we can tell he's a complex individual with deeper motives than just "Muwah haha" but the sheer apathy he displays at the consequences of his actions, which he is obviously aware of, makes him really creepy and unhinged.
2. Maleficent. Okay, this is fanboyism at its best since she's already my favorite Disney villain and she doesn't do much after II. Regardless, I love her in the games. Beyond her absolutely awesome voice acting that easily gets near the original, I love that she's still given some major importance even after the first game. She plays a big role in the Wayfinder Trio downfall by forcing Terra to steal Aurora's heart (which further fueled his guilt and desire to be stronger and thus fall more easily into Xehanort's clutches) and feeding lies to his friends about how he did 100% willingly, her preying on Riku kickstarts his character arc for the rest of the setting and she manages to be a thorn (lol) in more influential and important characters thanks to her manipulations and actions, even becoming an important asset in bringing down Organization XIII and forcing Xigbar to take some measure to hide the Black Box. Even after being relegated to the background, she maintains a certain aura of dread. I also love that she absolutely, utterly refuses to stay down despite all the setbacks she goes through. The sheer determination alone makes her impressive. The fact that she literally comes back from the dead in ways that should otherwise impossible by the setting's very specific rules only makes it more awesome. Besides, her boss fights are always awesome (my favorite are in BBS, especially the witch form) and even after II she proves she's still an extremely powerful and dangerous combattant no one should take lightly (hi Lauriam). And of course, the fact that she's a villain who openly admits she's evil and her motives boils down to "I want power, I'm evil and I like it", which I generally prefer over tragic and well-intentioned extremist antagonists.
3. Hades. Already one of my favorite Disney villains, I love him in the series for being both very funny and menacing at the same time. I find it awesome that, as the God of the Dead, he's basically unkillable even by the Keyblade. James Woods is of course unmatched in awesomeness, delivering a stellar performance that nothing can compare to and giving tons of excellent lines and moments. Needless to say, I'm extremely happy he's heavily implied to be in IV, especially if they give him a greater role in the general story of the setting. And let's face it, he has some of the best moments in the whole saga and the symbolism about him being the first villain to be defeated in III foreshadowing Xehanort's own demise is very well-thought.
4. Terra. I love everything about him. His struggle with his own doubts and guilt, his determination to protect his friends at all costs, the sheer power he possesses which even Xehanort recognizes. Contrary to what people like to claim and mock him for (hi reddit), he doesn't outright fall for the villains, except Maleficent and Hook, he always follow his own instinct and even uses some of them to get what he wants like with Queen Grimhilde. I really wish people stopped this because it's just infuriating in how wrong it is. His theme, designs and voice acting are extremely good and his fate makes him the most tragic character in the entire story for me but also serves to emphasize his absurd determination and willpower and how much of a badass he really is. I mean, the dude lost his body and animated his own armor to kick Xehanort's ass while he was technically at his prime out of sheer rage. This is absolutely awesome. And his return in III is simply the best of the "reunion" scenes for me. I just shrugged and said "That's cool" when Roxas and Xion came back but the moment Terra overcame Terranort and the Wayfinder Trio was reunited? I was outright cheering and yelled "YES!!!". I really hope he will appear more and get further development because he definitely deserves it. And let's be honest, Rage Awakening. Nuff said. Terra is easily one of the best characters in the story for me, above many others.
5. King Mickey. Most badass version of Mickey Mouse ever. His design is cool, his keyblades are visually stunning and his relationship with Riku is very interesting. I also love that despite being, well, Mickey Mouse, he's not some flawless, infallible figure. He has flaws and limits, sometimes outright fails even after a decade of experience and is not above needing help in his fights. His powerset based on light is also very cool in its versality (cutscenes alone give a lot of cool abilities) and I love the detail that he's very skilled at spellcasting due to having been taught by Yen Sid and I'm very glad Remind gave him the time to shine in his "Take down all the vessels by himself" scene while on the brink of death. The fact that he still has some clumsy moments also helps to keep in mind he's still the goofy Mickey Mouse we all know and love and is a grounded character with his own layers of personality.
Honorable mentions:
- Data-Sora: A particular case since he's very underrated as far as other selves go but I find him very interesting due to knowing he's a copy and having zero problem with it. I find it extremely interesting. Honestly? I like him more than mainline Sora or Roxas. Granted, I'm one of the only fans of the series who thinks Sora is meh good at best and Roxas isn't that much fascinating beyond his concept but still.
- Vanitas: Badass, menacing, extremely cool character and Keyblade design, very intense boss fights, awesome OSTs. However, the main reason I like him lies in his final scene in III. Namely, the moment where he rejects both Ventus and Sora's attempts to redeem him. The game makes it clear Vanitas could choose to redeem himself but he refuses. He is darkness (literally), he embraces it, he loves it. He doesn't have very deep motives or bigger reason, all he does is just him embracing the kind of monster he is and running with it. And it makes him all the more interesting when he basically states to Sora and Ventus's faces this actually makes him important to them, as he's effectively their shadow. My only grip is that he's relegated to a henchman in III and appears bit too late to truly shine among other villains.
- Aqua: Best female character in the series by far for me. Her design is very cool, her Keyblade to and as someone who loves sorcerer characters, I really enjoy her gameplay due to her emphasis on magic. She has personal stakes that made me very involved with her and her sacrifice at the end of BBS is one of my favorite moments in the series. 2.8 is actually one of my favorite games not only because it allows us to explore the realm of Darkness (and offers us some of the best OSTs, like The World Within and Forest of Thorns, and environments) but also because it further develops her character and her determination to keep going and fight the Darkness for the sake of her friends. And beating Terranort, aka, Xehanort at his absolute prime, in direct combat? Massively badass. A bit too bad BBS doesn't develop her as well as Terra and Ventus, particulary in regards to her judgmental side which isn't that much used in the story compared to her friends's flaws, and her vocal performance by Willa Holland is...shaky to say the least (she gets better later though).
- Terranort: Not really an excellent character since he's just there to be a jerk with immense raw power and does not have much of a personality aside from that but he works very well in that role. I just love his unhinged and over-the-top villainy (the vocal performance is excellent), dude is needlessly mean even compared to the other villains and is a MASSIVE powerhouse. And his boss fights, especially the one in the Final Episode, are some of the best in the series. Plus, he technically defeats Sora in Remind. That earns him a plus for me.
- Young Xehanort: YX is pretty interesting to me in that similary to Data-Sora, he's just an extension of someone else, a pawn even in his case, and doesn't mind his role. In fact, he even embraces it. For most of his appearances, he remains calm and collected which gives him a menacing yet almost robotic personality and I like that, among the main Xehanort trio, he remains purely evil and a jerk through and through. Bu tI think what I like the most is how they handle his powers over time in his boss fights. I like that he can alter time in so many versatile ways, it makes for an interesting antagonist without making him too overpowered.
- Xemnas: The best of the Xehanort variants for me. Spectaculary powerful, super cool design that perfectly blends Terra and Xehanort, I love his laser swords and his powerset based on the Void and his absolutely apathetic yet utterly cruel personality make him standing out. The man has a lot of charisma and menacing aura to spare and does it with delight and his boss fights are always awesome. Of course, there's his iconic, utterly spectacular final fights in II and his secret boss fight as the Unknown in I is a nightmare but still awesome and extremely creepy. His final scene in III adds interesting layers to his character as well. Also, Disappeared.