KH1, I've made threads about it and said it before. Kingdom Hearts 1 had structure, all the worlds blended into one another, the game had excellent platforming, a better simpler plot, not too much characters and events confusing you throughout the game. The game feels classic and original. The battle system may have not been the best but i actually had a challenge playing this game and Donald and Goofy were actually useful besides healing you. The World selection was a million times better, and I still love playing through these worlds time and time again, but not in the constant rehashes of them within Re:COM and Re:Coded. Anyone that thinks anyone of the games is better than KH1 clearly started the series late and cant appreciate good work, well that my opinion at least.
BBS, the characters, the story, the originality, and the connections to the original game made me enjoy this a lot, the world selection mostly the 3 Princess of Heart worlds, the Battle System, and the Villains made me love this game. It is basically one point away from being as good as KH1, it still lacks structure, i feel like your just going to worlds to go to worlds, nothing more. still its the only one since KH1 that i actually play over multiple times.
KH2, this game definitely does not have structure or good world selection, its not challenging, the plot is ridiculously twisted with characters and events, but for some reason i loved it, the epicness of the story, twisted story it may be, had me constantly wanting to see what happens next. the organization and there impact on the worlds was interesting, and revisiting worlds, boring as it is, was refreshing to no it lengthened the story.
Re:COM, the Card system is a more annoying and unfair version of the BBS battle system, but enjoyed it none the less, it had its flaws, mostly just returning to the same worlds, but it switched up the plots, and incorporated the Organization for the first time and made them interesting and mysterious.
Re:Coded, it incorporated the BBS battle system, the Days panel system, the KH1 and Re:COM worlds, and the hacking system and plot of coded, but managed to almost be worse than all of them. It another rehash of KH1 with KH1 soras design and the same worlds, used for nostalgia sake, but the game was alright and kept me interested.
Days, it is the weakest of the series, the mission system was not kingdom hearts at all, but being apart of the organization and seeing how they operate was interesting, the characters were not interesting, the worlds were the same as in KH1 and KH2, and they seemed less interesting, even Neverland which they added to.
Lets hope KH 3D can top BBS or even the original, and bring the good old Kingdom Hearts feel back. Im still waiting on KH3 to be the best game in the series