Some may disagree, but I always preferred the original Kingdom Hearts over its successor. Even at the time before Square-Enix started rereleasing all the games on PS3 and 4 I never managed to beat KHII more than once. In contrast the first game must be one of the games I've played from start to finish the most.
Self contained plot
Unlike all other entries in the series the first one still tells a story from start to finish that players can enjoy without having played any other game. In a timeline where Kingdom Hearts II tells its own story without relying on retcons of the former games' plot the series' overarching story woulnd't be the convoluted mess it is today.
Integrating Disney content into the plot
Disney worlds in Kingdom Hearts don't forcefuly retell the stories of the movies. Instead they use elements from them to create similar stories that aren't quite the same, but are better fits for the format of a game and that integrate somewhat into the main plot of the game as a whole. A side effect of Kingdom Hearts II doing the opposite is also that the game is loaded with cutscenes, constantly interrupting gameplay (something that's been an issue in the first game too, but less pronounciated).
Combat feels less manual
Something that changed since Kingdom Hearts Final Mix. The original game used to play less like a button masher and required more precision and elaborate positioning if you didn't want all of your attacks to miss their targets. I really miss that kind of attentive gameplay.
These would be fine with me if they were more optional and didn't remove Donald and(or) Goofy from combat. On the one hand I'm not a fan of how the game locks Sora's mobility abilities behind these forms, and on the other I feel Kingdom Hearts II strays a bit too far from its Disney x Final Fantasy crossover roots.
It feels like I'm alone on this one, but I love the kind of level design used by the first game. Some levels in KHII are just too straightforward, almost like out of FFXIII. Monstro in KHI was a bit confusing, but even that world is easy to navigate if you pay attention. The mini map introduced in KHII and beyond also feels unnecessary - I enjoy those games a good deal more with them turned off. Hopefuly that'll be an option again in KHIII.
But all that nitpicking aside I enjoy both games a lot. Kingdom Hearts II is really tough on proud mode and can be hard to get into when played right after the first game, but the game becomes much more fun after the insultingly simple level desing of its Mulan world.
Truth be told I this is a rant about an experience I had like 10 years ago when replaying both games on PS2. Always wanted to get this out of my chest. However Kingdom Hearts is still the better game to me though, after all these years and having replayed both games on PS4.