I think design and content wise, Kingdom Hearts II was the best. It had a great variety of Disney franchises, some sweet Final Fantasy cameos, and the battle engine was really, really fun. It did, however, had the "weaker" story. The first Kingdom Hearts I think had the tightest story since it focused its development on Sora and friends and the Disney characters he visited. Nowadays, the story is about whatever organization trying to do something regarding hearts, and whatever protagonist is available at the time trying to defeat them while having an existential, emotional crisis in the process... and it features Disney characters and worlds. Don't get me wrong, though, Kingdom Hearts was still unnecessarily complicated and it was very emo every once in a while. The difference is that it was all countered by the cheery, silly and fun disposition of Disney, so it all managed to balance out, hence why the game is more iconic and more fondly remembered than any of its sequels.