I wish I could say that it's tough, but it's honestly not. KH2 Xemnas has to take it home. I loved the KH3 final battle, and the World of Chaos fight from KH1 will always hold a special place in my heart as the first final boss I ever beat in any video game, but I've gotta join the crowd on this one. That final gauntlet against Xemnas in KH2 was just some next level stuff.
Personally, if I had to rank all the final bosses in the series, here would be my list:
1. KH2 – Xemnas. Absolute perfection.
2. KH3 – Master Xehanort (including the fights with Armored Xehanort and the Replicas). Not only was the arena the coolest looking in the series, but the whole fight is designed with this sense that it is in fact the culmination of all the battles you've fought thus far, and I also really liked Xehanort's defeat scene.
3. KH1 – World of Chaos. This boss blew my mind when I first faced him as a kid. The music to this day still chimes in my head, and I've always had a thing for fighting gigantic enemies in huge formless arenas. They said that WoC was originally supposed to literally be its own entire world at one point, and I would kill to be able to see what that would have been like.
4. KH3 Secret Episode – Yozora. Just too cool, man. This fight was so freaking cool. The music. Shibuya. Yozora stealing your Keyblade. The disco ball. The whole battle just sweated mystique, and I loved it.
5. KH3 ReMIND – Armored Xehanort. I'm also going to count the Replica battle before this one, because fighting them with each of the characters was just one of the coolest moments in the entire series for me. Plus, not only was the AX battle more intense than the base game version, but playing as Kairi was something I and many other people have waited over a decade to see, and baby girl played like a dream.
5. KHCoM – Marluxia. Fighting him in his first form was in and of itself a cool and intense battle, but then you switch to his giant forms, and everything just FEELS more climactic. Plus, the added context of knowing that the Nobody that accompanies him in that fight is his sister makes the battle feel all the more dramatic.
6. KH3 Limitcut Episode – Data-Master Xehanort. Technically, you could choose either this guy or Data-Xion as the final boss here, but I'm going with MX, 'cause c'mon. The fight was really amazing, but a little too intense for me. I'm fine with a challenge, but I'm not one of those hardcore challenge-hounds that crave to be bashed against a wall a thousand times. Not only that, but it is just a revamped version of the final base game fight anyway.
7. KH 0.2 – Demon Tide. This was a really cool battle to show off from a technical standpoint for what KH3 would end up looking like. The music was also notoriously catchy, and I loved the opportunity to have another fight in the destroyed Destiny Islands for the first time since KH1. That, and the moment when the sky shifts red and the music turns intense is just spine-chilling.
8. KHCoM – Ansem. This was a hard fight. Re: CoM made it a little easier because of the Duel system, but man, it was still tough. Also really cool to be the first final boss where you play as someone other than Sora.
9. KHDDD – Armored Ventus Nightmare. It's an okay fight, but what moves it higher up the list is the music and the setting. Not great, but there are definitely worse final bosses.
10. KHBBS – Vanitas. I give this spot the same reason as the AVN, namely the setting. It was really cool to actually have a final boss in the Station of Awakening. And even though the final portion of the fight was a little bit janky from a technical angle and not really explained well, it was still cool to execute once you learned how.
11. KHBBS – Ventus-Vanitas. A fast-paced fight, but it's basically the same as the other Vanitas fight minus the cool/janky final segment and cool setting. But at least there's Mickey.
12. KHBBS – Terra-Xehanort. I'm giving this same spot both in his fight against Lingering Will and against Aqua since they work pretty similarly. I really wanted to put him higher on the list, but I think the developers just designed him way too cheaply. Maybe it's just that I don't know exactly how to deal with him, but I found his mechanics infuriating. You can dodge projectile attacks OR heal yourself, but not both!
13. KHMoM – Master Xehanort. Even though we were all ready to be done with this geezer at this point, I still found his musical battle in MoM surprisingly intense. But it's low on the list because it pisses me off that we didn't get to fight him as Kairi. Major points off for that.
14. KH coded – Data-Roxas. I mean, it's just a less intense version of the Roxas fight from KH2. Next.
15. KH Days – Riku. Yeah, I'm counting Riku as the final boss and not Xion, because he just factually is the final boss, and if we start counting penultimate bosses too, then this list is going to get a lot longer. I think we can all agree, it was a pretty underwhelming battle, and definitely doesn't live up to its original incarnation in the KH1 secret ending movie.
16. KHX – Foretellers. I mean, I guess I feel kind of obligated to include these guys as the final bosses of the browser title, but, eh. Whatever. I guess you can give them points for being set against the backdrop of the Keyblade War, but I'm sorry, the whole cutesy aesthetic of the X games robs that moment of any intensity or weight that it could have had if it had been a proper console title. Plus, since I as an American was never able to actually PLAY this game for myself, I gotta put these guys at the bottom of the list.